The geometry is more upright, facilitating an aero position for the rider. The frame itself is more aero, which is a slight advantage. Most TT bikes are actually heavier, because wind resistence is more important than weight for most TT's (Tubes tend to be big and shaped for reduced wind resistence, but heavier in the process.) My tt bike is a heck of a lot less comfortable than my road bike, and I find that I have to make myself ride it when I'm preparing for an upcoming TT.