It's not just science fiction periodical houses that rely on it.

[Emrys]

Smaller independent publishing houses use it to promote their books because they don't have the advertising budget to make a splash otherwise, and loads of less well-known authors use it to build up a following and publicize their latest writings. It can't be matched for the intimacy that can build up between author and readers over time.

The OP's last tweet says it all.