There are a few reasons that come to my mind.
- Assurance of a Ticket: In the past, there were a lot of times when you go out to watch a movie with your friends only to see find yourself with the Housefull sign outside the ticket counter. But now Online Ticket Booking sites have made it quite easy for us to book tickets. No more queues and no more Housefull signs. But I have also observed that while most Multiplexes are listed in these online sites only a few Theatres in a city are listed. So it's quite natural that the advantages of online ticket booking go to the multiplexes.
- Multiple Choices and Varied Timings: Some time back most of the theaters only have a single screen and the show timings were fixed for all shows. So if you get to Theatre A to watch a movie and find that there is no ticket, there would be no use going to Theatre B since by the time you get there the show would have already begun. Multiplex meanwhile offers to you multiple movie selections and more screens.
- Pricing: I don't go out regularly to watch movies, just around a few times a year. So it doesn't make any sense for me to consider the prices for an activity that I rarely participate in, especially when the difference is not too significant. Besides, there are also plenty of online booking sites which offer discounts.