I guess it depends on the quality of the glass and who makes it really.... and of course you still don't wanna overfill your piece cause it could flood your bowl too which is worse than sucking up extra water but yeah that's the general idea, blocking the splash back so you don't get a mouthful of water
The last time I had a bong the splash guard worked fine. I've hit others though and got a little bit of a splash back. Really it depends how much water you use too.