SIMPLE...WE DON'T WEAR OUR HAIR WET!
That's reason #1, but there is a whole lot more that goes into it. When we are taught to cut hair we learn patterns, guides, and rules... but not necessarily how to customize all of that to highlight each client as an individual.
Only with years of experience, after our basic training, do we understand those guides can be manipulated, elevating them in a way that treats each head of hair as different as a finger print.
Cutting the hair dry allows us to take into consideration hair texture, cow licks, heavy parts as well as allowing us to see if one side reads thicker, if that pesky wave is only present in the back, if the ends would benefit a blunt or textured edge.
Oh, and don't get us started on those curly girls...you know you've asked for an inch and suddenly 3 got lobbed off. Dry cuts let us take that inch in your natural form. Straight wet hair doesn't suit you anyway.
So unless your an everyday surfer, swimmer or generally walk around with a wet head, dry cutting for your dry style is the way to go.