People see it as a way to spread awareness about the fediverse alternatives that are out there. Like "hey, if you like this, there's more where that came from." It's not for viewers who are already here, but for those where the post inevitably travels.
I dunno. Both watermarking and being annoyed at the watermarks seem like a waste of energy to me. If people are going to generate content, I'm not going to sass them about how unless it makes something about the content worse (harder to read etc).