Maybe someone is sending me a message? Or maybe not, and of so, why not? It only takes 3 seconds to respond to a message, what are they doing that they can't respond? And Amy sent me a message four minutes ago and I didn't respond because I want to know what Dustin is doing first, but I don't want her to think that.... AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA....
But if you are swimming or something then your phone is out of reach and there's nothing you can do so you just accept it and enjoy swimming and talking to whoever you are swimming with.
Also it mixes everything together, so if you just want to do something benign, you take out the phone and have a zillion other messages to worry about.