From the videos I’ve found, it helps your suspension by reducing pedal kickback (by adding slack between the crank and the sprocket like you mentioned).

Now if you’re an absolute MTB dummy like me, you may also not know how or why kickback happens, so here’s an article I found

Never owned a full suspension bike myself, only bikes with front shocks, and my trail riding has been very mild (not downhill stuff, only nature trails that were on my route as part of a bigger trip) so I’ve never put much thought into it. I guess it helps a little if you’re doing downhill type stuff, and is completely useless if you’re riding on roads like I tend to.