Do you have to buy tickets in advance

You can If you want to, is more convinient , however you can always buy the ticket at the front of the cinema

There is usually seats leftover, but if have a special place you like to sit I would buy in advance.

Not necessary

You can but you don't have too. If you are going to the 1st ever showing of a Movie I would recommend it because the movie might sell out when the seats get full, but otherwise, no you don't need to.