Anonymous remailers are nearly as old as Usenet is.
If you like Usenet and what can be found on it, you will love remailers.
If Usenet is sometimes too mùuch to cope for you, you will hate remailers.
Here is my own classification.

'Victims'

You have been the victim of an offence: a rape, an assault, a harassment, something against your rights....
Or you slipped into somme kind of addiction: alcohol, drugs, work, domestic violence...
Ypu are dow, your values collapsed, you cannot controml your own life and you are *ashamed*.
You do not know why, you know that you are not reponsible, but a victim.
It does not matter, first and before all you are *ashamed*, you want to hide and lick your wounds...
You would like to scream for help, enact retaliation, use every possible legal way...
But you cannot becausefirst and before all you are ashamed of what happened to you,
ashamed of what they did to you.
Anonymous chat on relevant NG will help you meet people who faced similar ordeal, or able to help.
When your wounds have scarred, when your scattered ego is rebuil, when you know your right...
you will come back in the open.
(English-speaking NG alt.recovery.xx, alt.support.xx, French-speaking NG fr.alcoolisme or fr.misc.droit...)

'Witnesses'

You witnessed some criminal act, or a touchy situation, or you heard of it,
but you do not know what to do...
By identifying yourself to seek help, you might point to the victim (who wants to hide),
or you might alert the perpetrator (who might flee or destroy evidence or attack you).
Ask for help and advice, anonymously, in appropriate NG.
After you collected appropriate data (relevant and competent support group / lawyer / police department).
and when you are ready to act in an efficient way, you will go into the open for good.

'Whistle Blowers'

Your duties put you in contact with some 'secret' potentially damageable to the public:
    health, safety, sweet deals, embezzlments...
In that conflict between your interests as an employee and a citizen, you do not want to sacrifice for nothing.
    how far is the hierarchy involved and covering up?
    should you tip somebody outside / which judiciary / the press ?
Better make a sure move to avoid destruction of evidence and retaliation.
You need a prudent and anonymous approach
  waiting for the differents apparels to gear in (hierarchy/administration/judiciary/media)
  waiting for a formal inquiry/hearing to start, in which you will take your full role in the open.

'Minority opponents'

You are the only one with your ideas: swan among ducks or midget among giants, it does not matter.
You will have to pay a *high* toll if they identify you.
Meet your kins or try to gain new supporters to your cause...
  waiting for the time you can speak openly and without fear.
  ot when you can cut heads off, once you conquered power :-(
(Cf: FAQ(Frequently Asked Questions)) about hottest political topics.)

'Celebs'

You are rich and famous, you are prominent....
....known all over the world.... or in your city.... or in the schoolyard.
Anyways, you have enough to comply with the image you got to show off
to your court, your servants, your fans, the paparazzi or Mrs Jones.
You want, without fear of autographs or endless comments, to be able to do what you please,
dig into new subjects (which did not make you famous) you like or would like to be involved with.
As a person, peer to peer, with aother persons. quite simply.

'Specialists'

You are outstanding in your domain. You are the leading light, respected and unchallenged.
Your aides reverence you, yiour students worship you.
Unfortunately, on that terrible off day,
you cannot remember at all the intricate details of that problem which you normally master so easily.
  like that famous navy captain who could not tell port from starboard...
Discreetly refresh your learnings...

'Debaters'

You are preparing a conference, or a thesis, or a memoir which will get you the Nobel.
Waiting for your 'true' audience, you would not mind testing your ideas on some consenting panel,
without prejudice, special obsequiosity or animosity.
Check if your ideas stand by themselves, without your notoriety.
Or start a deliberately provocative thought (opposed to your views) to find new material.

'Misanthropists'

You might have your own ides about everyrhing.
But you do not want to start a war about it with your neighbour or your grocer.
And you do not wish either some Usenet sicki to come around and jill your dog
  because your recipe for apple pie was different from his.
Or that a MMF-frenzy floods your mailbox...

'Business as usual'

Your business is nobody's business.
Well: you mean everybody is spying over everybody's shoulder.
We do not leave in an universe with gentlemen only.
As long as the contract was not signed, competitor may take over. Let us be prudent....

'......'

'Solidarity'

You do not belong to any of the afore mentioned categories, but you agree they are fully entitled to that behaviour.
In order to allow them to melt in the crowd, you will use for yourself the same encryption techniques,
  protecting both your identity and the content of your mails to generate dummy traffic for the others.
The same you would slip your snailmail in an envelope.
Or you would refuse for a job interview
to discuss your sexual orientation, your religious or philosophical beliefs, your race....

Trying not to give bad ideas to anyone,
let us just say that any message whose main (sole) purpose is to annoy or harass the recipient is not acceptable.
Same for mass mailings, repetitive or identical messages.
Safeguard steps are taken to minimize / prohibit such annoyances.
(Cf: How to shelter from Abuses?.)

A number of loud mouths decided to give anonymous remailers a bad name.
Whether they feel threatened by 'legitimate use', or they were former victims of 'unacceptable use',
or the mere concept of anonymity is beyond their intelligence, it does not matter.
In general, they are agressive, bellicose and rude characteres, soldiers without a cause.
The same will harass a NewB because of a signature too long, a misplaced quotation,
  an article improperly directed or multiposted.
In the name of 'purity in the news', they are ready for a holy war,
  and they endlessly refer to a Netiquette whose basic rules they blatantly disregard.

It is my belief that these self-appointed vigilantes follow provocative tactics
and are responsible for worst excesses
-- *massive* multipost of illegal ads (CD Warez, weapons, drugs) or hate or racist speech
-- death threats
-- encrypted (garbage) mail in non-dedicated NG.
-- mail-bombing targeting mailing lists
-- besides 'flood' attempts.

Those (ab)users are not protected by the privacy policy.
I will use any way I feel fit
  -compatible with other user's security
  -including legal action
to push them off the scene.

No privacy, no sanctuary for the ennemies of privacy.