I have the heavy duty one from Just Straps.
+1.
As the others have said, they're simply to spread the load between two points on the chassis. For monocoque cars (like our Pathfinder), this is A Good Thing. Some R50 owners have had recovery points torn off the vehicle by a heavy snatch recovery, when a equalising strap or bridle wasn't used. Part of that was caused by the design of the recovery point, but the fact remains that using a bridle between two points greatly reduces the load on each and probably wouldn't have been a problem.
As Burnsey says, never use a shackle to attach a snatch strap to a bridle.
As for how effective they are, and how well they work at an angle, I'm not sure how you'd even measure that. You certainly wouldn't be worse off by using a bridle in any circumstance I can think of (not to say there isn't one, just that I'm not imaginative). For the cost of them, and the extra 10 seconds they take to set up, I'm happy to include it in my gear.
Good luck!
Matto