Saw a display for pre ordering a digital copy of the force awakens. What is the purpose of pre-ordering if it has infinite copies?

Much like Video Games, Pre-Ordering is a gimmick and nothing more.

You can watch it at midnight.

If you want the physical copy (assuming that the store has a midnight release) then you need to go to the store, wait in line, pay for it, drive home, and then watch it.

If you order the virtual copy now then you have to wait for the movie to download before you can watch it. With so many people downloading at once there might be crashes or the speed will be slow. So instead of getting to watch it at midnight you might be able to watch it later tonight assuming that you have work or school.

To feel special.