Exactly. when you open, you want to open all the theaters, so there's a rush. Then most movies are close to the same run length - maybe a 90 minute kid's movie or a big 3 hour blockbuster, but most are around 105m-120m. So there will be ~30 minute span where the movies are getting out and the crews are cleaning, then the next block starts ~10 minutes after that. So another rush. The manager is trying to keep each theater playing as many times as they can during operating hours.

The main reason to break from this model is when you have a long movie playing in multiple theaters. Like when Interstellar came out and the 15 theater plex I worked at had 4 theaters playing it including the IMAX so you could get a start time every 30 minutes or so.

Edit: more accurate runtimes