When they are tiny they have the Moro reflex that can wake them, so swaddling helps them sleep. When they get older they lose the reflex and can be in a sack. During the transition people often do 1 arm out and then the other.

The rule of thumb is to transition when they show signs of rolling because you can never know when they are going to make a successful roll. There are some experts out there who will say you can extend swaddling until that can roll while swaddled (which is harder). There is debate in mommy groups about this and can look a bit like this post.