되다 has a few different meanings: 1. to become something, 2. to come to a certain action/state, 3. to work out.
-게 되다 has the second sense. It means to end up doing something or coming to a certain state without intention.
1. 안 먹고 싶어요. 그런데 먹게 돼요. I want to stop, but I still end up eating.
2. 지난 주에 영화를 보게 됐어요. I got to see a movie last week.
3. 그렇게 하게 될 거예요. (I/you/they) will probably do that (eventually).
4. 어떻게 여기에 오게 됐어요? What lead you to come here? How did you end up coming here?
#2 sounds slightly unnatural because -게 됐어요 is a bit too big a phrase to use for something as trivial as watching a movie. We are more likely to say 지난 주에 영화를 (하나) 봤어요.