If you ride road bikes, there is a good chance you want to go faster. You can see those KOMs on your computer, and you want to demolish them. You want to destroy your competition. You want your name beside that crown.You can train harder, or you can train smarter. Part of training smarter would be to buy an aero bike.
An aero bike will not transform you from a Cat 4 racer to a Tour De France hopeful, but it will give you a legal injection of speed. Ignoring the aerodynamic boost for a bit, one of the main reasons for the speed boast is that aero road bikes are stiff.