A big part about it is the ego stroking. I'll bet there are a lot of girls who are technically unavailable that like to hear their phone going off with messages from salivating men. Not sure now they would hide it, but sometimes you have to wonder.
It probably happens the other way too, but I don't spend time looking at dudes profiles, so I don't really have an opinion.
My ex did something similar near the end of our relationship. I actually caught her in the act, but I had no way to confirm either way.