As the rules say:
18. Having multiple accounts and account sales are allowed, but account sales are discouraged.
So, having multiple accounts is allowed, but that doesn't mean you can create multiple accounts, misuse them, and get away with it. If you are caught doing that, you will have all your accounts red-tagged by DT members and they will basically be useless since they won't be trusted anymore, nor you can join any promotions, bounties, or signature campaigns with them.
You can even enroll your main account and your alt account in campaigns as long as you are not cheating them, which means you can't enroll your main account and an alt account in the same campaign and this is written in the rules of every campaign you will see.
Why would anyone want an alt that everyone knows is an alt?
To use it on a different device, maybe. Reputable members that have alt accounts:
1.
theymos - alt:
theymos_away2.
LoyceV - alt:
LoyceMobile3.
hilariousandco - alt:
hilariousetc4.
tranthidung - alt:
dzungmobileThere are probably more, but I can only think of these at the moment.