Do you mean thickness tapering of wood in the limbs? If so, it allows for even amonts of flexing/strain to be distributed along the length of the limb. Without tapering of thickness and/or width, a limb will do most of its bending near the handle, which can cause overstrain there, and possibly unnecessary handshock, set, chrysals, limb failure, delamination, etc.
The taper rate helps determine limb action. A slower taper rate bends more near the handle. A faster taper rate causes the limb to bend more out toward the outer limb. On board bows, backed bows, trilams and such, limb taper can be accurately done(ground) prior to tillering if one has the means. I grind them just like I grind tapered lams for glass/wood laminated bows... with my lam sled and thickness sander. It's not necessary, but it's very accurate and expedites the tillering process on such bows.
If that doesn't answer your question then I may have misunderstood what you meant, so please clarify.