The benefits of incorporating strength training into your regime are well documented. As an athlete, being physically stronger means your sub-maximal efforts become easier, resulting in more speed, greater power output & improved reactions times.
For endurance athletes, if you could be twice as strong, carrying your bodyweight around a race would be much easier and therefore would require less energy.
For speed based athletes, improving your ability to recruit motor units (nerves and muscles) will result in greater power development.
We also see a reduction in injuries, as you condition your body to the effects of repetitive training.