Running is often described as controlled falling, and this is for a good reason since the body uses almost half of its energy to prevent you from falling. With each step there are forces acting from different directions, forcing your body to collapse. To prevent that, the body relies on specific muscles that stabilize your joints and allow prime movers and antagonists to do their work.
To improve your running performance, it would logically make sense to focus just on running and increasing your mileage. While this strategy might be beneficial for building endurance, if the muscles are not strong enough to absorb this extra stress from running, stress will get absorbed elsewhere and will eventually lead to injuries.