Originally Posted by jock

The sloping top tube is the side effect (not the reason).

The triangle created by the seat tube, rear stays, and chain stays is smaller and closer to an equilateral triangle. By doing this it creates a more rigid support for the drivetrain, which means pedal drive goes directly to the wheel. The theory is correct, but it can be compromised by poor choice of materials.

Lesser effects are slightly reduce frame weight (when comparing frame built of the same materials) and marginally lower centre of gravity.