The stud girth is to protect the horse's chest from an impact with the studs in the forefeet. It is relatively rare (or so I've been told) for a horse to strike himself with his feet on jumping, but he can hit himself with the studs if he is a tad over enthusiastic. Once you reach 1.20 or so with studs in you do need to keep a careful eye for stud injuries. Personally I tend to use a stud girth if we're competeing over a metre with studs in; better safe than sorry!
I always jump mine horses with a stud guard regardless of whether they have studs in or not. You would be surprised how many marks there will be on the stud guard after you have jumped.
