It's all about balance. There are a couple of different aspects..... some folks, usually stock removal makers, will taper the tang. Those who forge will tend more so, to install a distal taper (thickest portion of the blade is the ricasso area, and the blade tapers fore and aft from there)
In the case of just tapering a tang, it lightens the handle end, and will often give the knife a "lighter" "faster" feel. Those same things apply to a full distal taper, but the tapering of the blade also lends additional benefits.