What's the deal with assigned seats? Our friends got there early but then had to move right before movie started because of somebody's assigned seats.

It also assures you a seat when you have paid for your tickets. They ask you at counter to pick an area where you want to be seated. Also, it endures you seating at brand new movies.

All seats in the theater now are assigned. Your friends had to have picked seats when buying their tickets unless they sneaked in, meaning they had their own seats reserved. If they sat in someone else's seats then they had to move because those seats were reseved by someone else when they bought their tickets. This ensures that people aren't wandering around looking for seats and prevents movie hopping.