Having the mast further aft has two advantages. The centre of effort is lowered slightly reducing the chances of capsize and also the sloping luff of the foresail results in a component of force producing a lift at the bows. This counteracts the bow burying tendency of some cats which have very slim bows that tend to give inadequate bouyancy. Prouts have provided adequate bow bouyancy as well as the sloping luff so they tackled the problem twice.
Capsizing a cat is generally considered to be a very bad idea. One of the cruising cat manufacturers (Prout?) adopted this mast aft/low rig/big sloping genny configuration as a way of keeping the centre of effort low, and reducing the heeling/capsizing lever.
I must admit that I've no idea whether it turned out to be an effective sailing rig, though I haven't seen anybody imitating it.
They sold a fair few so either it worked, they fooled lots of people, or they invented a new kind of motor sailer.