I have been shaving my eyebrows off for quite some time now, years even.
But still, when asked, I couldn't tell you WHY.
Maybe because I like the look, though half the time I don't bother with any.
Maybe because I can't be bothered with weekly plucking... although I end up having to shave them most days anyway.
What I CAN tell you.. is that eyebrows are there to stop sweat running into your eyes (learnt that the hard way!) and help with displaying readable body language.
I have never been able to do the quizzical eyebrow up, eyebrow down look. Of this I am quite jealous.. atleast now I can draw them that way and spend the day making up for it.
You have to be careful though, just because you don't do mornings, doesn't mean you want to spend the rest of the day looking like you hate the world.
Oh and then there is the hazard of mixing drawn on eyebrows with drunken friends.. they sort of want to use you like an etch-a-sketch, the results are almost always disastrous.