The bell guard plays varying roles among the three weapons.

In the case of foil, the bell guard is largely useless, since your hand is not target area anyway. It serves mostly to keep your opponent's blade from smashing into your fingers if his blade slides down yours.

In the case of epée, the bell guard is quite important as it serves as a shield for your hand and (assuming you've got a decent stance) forearm, both of which are target area. The guard is also large enough to use to parry with, in a pinch. In some cases you can tell how much use an epée has seen by examining its guard. Relatively new weapons will look like glossy metal with many scratches, whereas old weapons will have what is virtually a matte finish.

In the case of sabre (and I apologize if this is inaccurate, I am not a sabreur) the guard helps to protect the arms, which are target, and plays a large role in defending the four line, since that is the area where the guard curves down across the hand to the pommel.