For both compound and recurve, a strap helps newer archers learn proper shooting technique, as well as building both strength and flexibility for shooting. It sits comfortably on the bow hand and allows a lot of pressure you would feel when using the bow. If you are already shooting, it helps you refine your technique.
Recurve archers use a strap to help learn to use the back correctly, as you cannot overdraw, making the back do the work instead of the outside of the shoulder.
Compound archers need to add a small loop of release rope or cord for the release hook. Use it as a warmup or training tool. It helps you to learn and reinforce proper technique, as you can feel the ‘wall’ and pressure it as strongly as you do with the bow.
The important time to use a strap with a compound is when learning to use a back tension/hinged jaw release. Using the release with a strap for initial learning is an absolutely safe way to practice proper release hand position and control. Once mastered with the strap, it is much easier – and much safer – to move to the bow.