Since 2016, I have only seen these reasons-

1. To scam.
2. To participate in campaigns/bounties.
3. Get into trouble using their primary account, then use alts to turn the direction of the discussion to something else.
4. To shill.
5. Targeting a reputable member: I've seen a case where someone reported a member on the forum for Fake Plagiarism. The user reported getting banned, and theymos verified the source and found it was a setup and unbanned after finding it out.
6. Fake transactions, fake service vouchers, fake discussions on the service thread, praising the main account, faking deals, the list's long.