> People who do this are generally trying to be polite by not jumping right into the request

Sometimes it's the opposite and used as a trap to force you into answering something awkward on the spot - when you reply "yes?" in chat, they know if they ask something quickly after you'll feel obligated into replying immediately. Obviously I hate games like this but I've worked with others that complain about e.g. managers doing this often so they learn to absolutely dread "hello?" messages even when it turns out to be nothing.