I don't use draw reins at the time, but while my horse is out of hard work, I put him in the side reins, and they help a bunch. He is easily back sore, but chooses to giraffe his head and cause his back more strain, so the side reins give him limited range for his head unless it's downwards, which makes him work through his back and use his entire body instead of bracing. It relaxes him, and helps further by teaching him to keep his head down.
I rarely use side reins, but we use draw reins pretty frequently. It's really important to make sure that the horse is still using it's hind end and not dumping themselves on the front. Draw reins can be really damaging in the wrong hands but really helpful in the right ones.
Depends what horse - I like them there very helpful
I personally think of draw reins as guide for a horse learning that they can come down if that make sense. I think they're very front focused and they don't have a lot of impact over the hind end which is the most important part of the horse to me. i don't think draw reins are evil but I do think they can go very wrong very fast and should be used by a professional or under the supervision of a professional.
As for side reins, I have very little to say about them since I don't really use them.