It's about having a stable base. Would you squat on an elevated platform with springs to allow movement? Would you deadlift while balancing on a skateboard? For the purpose of building strength and size, there is no benefit. Granted I'm sure you'll here things like "I feel this isolates my pecs more" or some rubbish. Isolation is largely crap unless it is specifically needed. The bench press is not a great pec exercise despite the solid results most see in that area from getting better at it. The benefit of the bench press is simply heavily loading a large portion of the body's musculature (and the pecs are in here) through a fundemental range of motion. So, in putting those feet up, you defeat the real purpose and fundemental value of the exercise (i.e. ability to lift heavy weight and leverage more of the body's musculature). Granted I'm sure the bodybuilding world can come up with a whole bunch of creative extrapolated reasons they might want to employ this but I'd venture that in truth, the number of times it might actually be beneficial is generally very very small and the frequency with which it is used is far far higher than it should be.