Just out of curiosity, why are there often "bots," such as Majestic-12 [Bot], sometimes shown as online registered users?

I haven't seen that one in particular, but I often see Google's webcrawler bots showing as online, but they're just indexing stuff for the search engine.

After some quick googling, it appears Majestic-12[Bot] belongs to a distributed search engine at http:/www.majestic12.co.uk