The concept is simple: you know what you need to do, and you don't want to have to watch the screen whilst you do it.
You can use the simple timer to run through reps with fixed break and activity intervals, or you can schedule out a sequence of different exercises, using voice announcements to prompt your next activity.
StretchPing can be run in the background too, allowing you to use your phone for other tasks whilst you stretch.
The feature set is focused around there being minimal fuss between you and your exercise; no adverts, opinions or upsells, just your stretches and the app's pings.