We evolve from animals that manipulate the world with their mouths and use their forelegs for locomotion. I'd expect that using hands to manipulate things would be in some way tied to our mouths, it's not surprising to me.
I recall a theory that we do it because of our arboreal lineage. When apes moved into the trees to get away from predators, they would often carry things in their mouth (using their tongue to hold them) so leaving their forelimbs free for tree swinging. Hence the connection - if the hands are occupied, the tongue comes out.
