Definately. I managed 2 of the local theatres for three years before going to work for the bank. Depending on when the movie comes in from the distributor (usually 2-3 days prior to the release date), the movie is built up (some still use a reel by reel, but very few as most use the platter system). All of the reels of film are built and spliced together to form one giant movie, and thus put on the platter. The purpose of screening them is so that the person who built the film can make sure it was built properly (no bad splices to cause frame jumps, all the reels in order, etc etc). Of course, many theatre employees are invited to watch it too ( I guess as an employment benefit).
Best bet is this: Become friends with any theatre employee, and you'll see movies before they come out for free.. unless the management of that theatre are asshole about it. I used to let employees bring 2 or 3 or even more friends along, as long as nobody did anything out of line.