Compact frames are stiffer because the top, down and seat tubes are shorter and because dropping the top tube and seat stays down the seat tube brings them more in line with, and allows the tubes to more easily resist, the significant torsional (or twisting) loads imposed on the frame between the head tube and the rear axle.
Compact frames are more comfortable because the longer seat post provides a little more flex.
They have the potential to be more aerodynamically efficient if an aero seat post is fitted, the thighs are a high pressure area (fnaar) and reducing the frontal area/ drag co-efficient of the seatpost can provide huge gains which cannot be achieved with the short seatpost of an horizontal top tube stylee frame.
