People often think and feel several things at one time. Even when I'm totally focused on my work, I can feel the shadows of other thoughts skittering around the edges of my concentration — or perhaps I might be having a serious discussion with someone about a book that we've both read, but another part of my mind is busy imagining him naked or worrying about nuclear waste buildup. The extra eye lets me show several expressions on one face at the same time. That's one reason.
Another reason for the extra eye is that occasionally I like to force my brain into questioning itself. There's a dislocated headachy feeling that this produces. Your brain is trying to square what your eyes are seeing (a human with three eyes) with its (your brain's) assumption that humans have two eyes. My cockamamie theory is that periodically forcing your brain to question its most dyed-in-the-wool assumptions (even in this small way) is a useful exercise. It's our ingrained assumptions — the ones so old and so deep that we don't even know they are there — that are the worst enemies of logic and insight.