There are two different classes of user. One class checks those things and knows that their post didn't go through. (That doesn't apply to voting, though—that's important.) A second class of user doesn't check and doesn't know. That second class tends to be more naive spammers or promoters, and shadowbanning works well in those cases.

It's useful with the more sophisticated class too, though. If they have to start fresh with new accounts, it slows them down and makes what they're doing more obvious to the community.