Running consists of moving your arms and your legs in a synchronized forward motion. Movements of your arms and legs in this forward direction is called moving in the sagittal plane. If all you do is run, your muscle movement in the sagittal plane is awesome, but if for some reason you suddenly need to move laterally (sideways) or twist or rotate when you're running, your muscles may not know quite how to react.
You may be wondering when this would occur. It can happen at any moment. If you step off a curb and land the wrong way, or maybe you're having to jump over a pot hole, or maybe you land it a pot hole, your body can suddenly be shifted from that comfortable forward movment into a sideways or rotational movement.
Everyone's familiar with the main leg muscles (calves, quads, hamstrings, and glutes), but there are also tiny little stabilizer muscles that assist with balance and support. However, just like with any muscle, if you don't lose it, you tend to lose it. So, if you rarely move in the frontal plane(lateral movement, abduction, adduction) or the transverse plane (twisting or rotational movement), those stabilizer muscles may be either too slow to react causing a fall, or that slow reaction (or no reaction) may cause the other "main" leg muscles to over react resulting in a muscle strain or tear.
Doing a weekly workout in which you move your body in the non-traditional running planes will result in increased strength and balanace in your main leg muscles as well as all the little stabilizer muscles.
Below is a video clip of a simple (yet challenging) workout that's great to do once a week. It even makes a great dynamic stretch before a long run.