For high-speed things like running drill, heavy trail riding, or trick riding, I use a back cinch for extra support to keep my saddle in one spot and not tilting at all. For normal training though, there's no need.
Is that also known as a "bucking strap"? If it is, I never use one. I had a saddle once that had one and I used it then but none of my saddles even have one.....
No, bucking straps are used on broncs in rodeos. A back cinch is used to keep the back of the saddle down when you rope etc. I use one because it's there and I like it. It also has a connector strap that hooks to the front cinch so that it doesn't get back far enough to become a "problem". ;)
As you can see the back cinch is not located at the flank. That is where the bucking strap would be.