This practice, in addition to being prohibited and not encouraged here on the forum, is usually for the wrong reasons, such as scams for example.

I'm sure it's not just for posting images or other restrictions.

Why?

For official reason like to be able to post image and reduce newbie restrictions? Buy copper membership. Check below link for it:

https://bitcointalk.org/index.php?topic=2385104.msg24371150#msg24371150

There are some reasons not to buy an account, old accounts could have plagiarism in the last and this can lead to permaban if known after you bought the account.

For other reasons, better to register new account and post good and rank up.