I suspect lots of different reasons but for me I think it is simply catharsis to engage with something that doesn't require effort or reaction unless I want to, I can just consume. This helps my brain recover from the focussed deliberate work I do during the day.

Of course, this does beg the question, if we took steps to stop doing it, would it give us more time to do useful focussed work or would we instead just watch TV, read a book, go to the part or whatever.