For the last couple weeks I've been seeing a consistent stream of 15-30 invalid login attempts per hours on my site. Many of them are non-sensical usernames that nobody would ever register for real, and often contain typical spam-related keywords. They all come from different IP addresses so I can't just IP block/throttle the requests.

I'm not worried about unauthorized access to real accounts since they aren't using real usernames. And if it were a member of my site trying to brute force logins, they could easily scrape the valid usernames from the site, so I'm not worried about that kind of malicious behavior either. But what's the point of this type of activity? What would whichever bot operator is doing this have to gain by attempting all these logins?