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?

it's mostly the water that is being absorbed back in.  Otherwise we would dehydrate very easily as a species.