I don't over tighten my cinch. So I like a breast collar to help keep the saddle in place. Some of the people I let ride my horses don't always stay centered. I'd really hate for their saddle to slip under the belly of a hrose.
And we do ride a few hills around here. I happen to like the Beta or Biothane breast collars. They are easier to clean up.
I probably should use a crupper also. But I just never have.