To be able to filter with purely technical (ie. number of identical or
substantially identical posts) rules. Without any need of someone
"judging" the content itself.
> I mean, pretty much anyone knows spam when they see it.
Yep, you're right. OTOH, what will happend if someone deletes content
other folks do not consider being a spam ?
> posts a message that says "Get cheap lottery tickets now!!"
Let's imagine you decide to allow users to cancel what they "see" as
spam. Someone asking "hey, is this lottery legal ?" could be cancelled
in a legal questions group even if the question IS on topic and the guy
not spamming at all and wondering if the message he received by mail is
a scam scheme or not. And on the other side a clever spammer would just
post a dummy messages in cooking groups, asking "What's the best way to
cook a chicken ?", with a "Get cheap lottery tickets now!!" signature in
each messages, and that would be quite difficult to judge.