- Participate in multiple campaigns on the forum. This is allowed, providing you are ethical (i.e. the accounts dont participate in the same campaing simultaneously, thet dont merit each other, they dont create fake conversations amongst themselves, etc.).
- Distinguish when they are using the main account from another one from which they participate in a less demanding manner (i.e. using a phone with fat fingers will normally lead to shorter elaborated posts, and that may differ from the style one wants on his main account).
- Participate in a conversation without unveiling their main account, for fear of retaliation against their main account.
- Try to bypass bans (which they are not allowed to, except for appealing on Meta).
Now obviously people do create multiple accounts for the wrong reasons: trying to scam, bumping services (not really worth it now on the boards where it makes sense), creating a farm of accounts that merit each other and/or participate in the same campaign simultaneously, creating account to try to sell them off later on, etc.
I think another important reason for an alt account is, that not everyone has a secure internet connection everywhere they go. Using an alt account can mainly have security purposes. Would you like to login with your 5 year-old legendary main account from a public wifi via a mobile phone? Sure not. Using a lower ranked alt account, could be a good alternative.
Nevertheless you are right, most spammers/scammers use alt accounts to bypass bans and to enroll in most bounty campaigns as possible.