The benefit is you get a bow that is the poundage you want at what you actually draw it.

Of course you can also order the bow at x pounds at 28", allowing for 2-3 pounds (typical, ask the bowyer about their bow) per inch over/under what you actually draw. For example, you draw 26" and want a bow that draws 50# there you could order a 54# at 28" bow (assuming the bowyer says 2# per inch) and get what you want.

Two ways to skin a cat, but it still ends up naked

While you're at it, if you're ordering, ask the bowyer how they specify and measure draw length and match his method, as there is variance.