The foremost factor is obviously time and chance. Say you want to see the movie at a certain point of time but it is a common factor that you have to return from the cinema hall without watching the same. Of course, all tickets have been booked earlier and not a single is left for you. it is really irritating and you come back in sad mood. But why are you thinking so much when you have chance to watch the same movie online? In fact, it is the best way to see any movie you like at any time without thinking for availability of tickets.
Also, you are saving a good amount of money when you are watching the same online and for free. There are many good movie sites that offer free watching movie online. You can watch Mission Impossible 4 movie too at free of cost taking the same enjoyment and in much relaxed way.