[metmom]

A bench could work that way too. If you’re just looking for support of your shoulders for the torso workout, it would be much more stable and safer.

[thackney]

—— it would be much more stable ——

Without trying to recommend it, but just to explain why other people use them:

The unstable nature of the ball is the reason they are used. It causes you to tighten up many other muscle groups at the same time.

This one is claimed to be burst-resistant up to 2,000 lbs.
http://www.power-systems.com/nav/closeup.aspx?c=21&sc=69&g=2943&Power/VersaBall®

I do not weight lift with a ball but I have used them for simpler exercise.

The one I like best is to replace the chair with the ball. You more naturally move around and lightly exercise many muscle groups while Freeping.

[ItisaReligionofPeace]

It has to do with using the stabilization muscles in addition to the normal pressing muscles.