The more illuminating question is "what is the need for anonymity outside of illegal activity"?

One answer is that you might someday like to say something which some other party would have a problem with.

We might say that if you truly believe in what you're saying, the principled course would be to say your piece and let the other party be upset. But if the other party has a lot of power (for example a parent, an employer, a university, a government official, etc) they can threaten retribution for your words. If the threat is serious enough, you will likely decide not to speak.

The ability to say things anonymously removes the threat of retribution. It helps people speak who otherwise wouldn't, increasing the range of perspectives and information the rest of us have access to. This doesn't mean that it's beneficial in every single case, but on balance it's a social good - in fact, it's a necessary component of a free society.

Plenty of reasons. The most obvious use that comes to mind is to avert government censorship. Then there are whistleblowers. There are more reasons to desire anonymity than illegal activity.

Edit: also, ISPs can sell your browsing information now: https://www.usatoday.com/story/tech/news/2017/04/04/isps-can...

Even perfectly legal activity can result in undesirable retribution. For example, some rich company could decide to sue a person in response to a negative review. Whistleblowers will almost certainly face retaliation if their anonymity is not guaranteed. Anonymous discussion boards are inherently more free: because there is no identity associated with opinions, there are no consequences for being wrong or politically incorrect and this leads to people speaking their minds without fear.

Also, not everything that is illegal should be illegal. Civil disobedience is impossible in a monitored society.

Journalists, whistle blowers, people in oppressive countries, unjustly censored information, the government, etc.

The Navy didn't Tor for criminals, so obviously there is a use case beyond that.

Sometimes illegal activity is an ethical necessity.