Why is the rectum so absorbant?

Ok, I know it sounds weird, but I'm completely serious with this question.  Why is it that our rectums (or our large intestines in general) so readily absorb nearly anything they come into contact with (think suppositories, alcohol, etc)?  It just doesn't seem to make any biological sense.  I mean, the only thing that would naturally pass by that tissue is shit, right?  What could our bodies possibly hope to find in lumps of stuff they've already digested and deemed necessary to rid themselves of?  Is it just a last-second effort to extract any leftover nutrients that may still remain?

The large intestine absorbs water and bile salts (bile salts are used in splitting up fat globules)
I suspect that suppositories slowly dissolve and are absorbed along with the water, and that whatever the suppository was takes action somewhere along where the water and bile salts are transported.

like >>2 said, if it wasn't absorbed, we would dehydrate very easily. Really, if it wasn't absorbed, we'd always have diarrhea.