I think the major reason for KYC is for legal reasons.
The thing is, there really aren't any laws in any country that requires it for crypto yet.
And that's the key word: yet.
So although KYC is currently totally uneccesary, there is a high chance that it will get more regulated and possibly require some form of KYC in the future.
To protect against that, projects have KYC all the way through.

Also, it can help weed out scammers.
First off, it can make casual bounty hunters who aren't serious go away.
And it can help multi accounting.

But, despite all this, my own personal opinion is that it is totally unnecessary.
Every country that makes a new law doesn't do it retroactively, meaning, you aren't held accountable for things done before the law was made.