Catholics, why is the "blood" withheld from the celebrants at some masses?

I was raised Catholic and I couldn't tell you why, Take your pick, any answer will due, how about because they said so! It doesn't even make sense to Catholics, If you don't believe me just wait a little while, I doubt you get anything resembling a sane answer, or even better ask a Priest, that ought to be good for a laugh! Ask two Priests and I'll bet you will get to different answers, they just made it up!