I really believe that some do it for their bad internet connection, it has already happened to me, others do it by habit, but there is a small group of players that do it by strategy, when playing like this, you keep the calm a little more, you have a little more time to think whether you bet or not and how much you are going to bet and apart from that you notice that players have their play done, they are more desperate and you know that if you take them out of their concentration they are easier to defeat at the table. This is a good strategy to get to know the kind of players in your table.