I suspect it's a courtesy thing: "You've been kind enough to follow me, so I'll return the favour."

I find it difficult to keep up with the couple (or so) hundred I'm already following, so I restrict it to those who, after investigating them, I find interesting enough to add ... or else with whom I have a number of positive interactions over a number of subsequent posts ... rather than people who (for some weird reason) decide to follow me (based, prsumably, upon a single post/reply somewhere(, but then never actually interact with me afterwards.