This is a pretty simple concept in Strength training, but one that is easy to neglect, so I wanted to take a minute to look at a couple of ideas.
The first is that muscle groups work opposite each other. For example your biceps bend your arm and your triceps extend your arm. Or rather one set pushes, the other pulls.
Balancing push / pull exercises is very important in preventing injury and maintaining a healthy body. If one group gets disproportionately strong in relation to it's opposing group it's only a matter of time before something gets injured.
The second concept is that outside of a gym environment most "real world" activities are not straight pushes or pulls, but involve rotation. Throwing a ball, swinging a bat / golf club or even shovelling the driveway are rotational movements where one side pushes, the other pulls.
Again, this is a common cause of injury if the muscles involved are not trained for this. So not only do pushing and pulling exercises need to be balanced, but training those muscles to work in more dynamic situations is also important for injury prevention.
And the third part of this is stabilization muscles. Or rather the muscles that help stabilize you're main muscles as they do heavy work. To see what I mean compare doing a pushup on the floor vs with your hands on a exercise balls or a suspension set up (gymnastic rings, TRX, etc.) I would refer to this article, for stuff that i needed for gymnastics.
Imagine taking a race car engine and sticking it in a economy car without upgrading the tires, steering, breaks and weight distribution... It's a crash waiting to happen.
Same thing when training, a large number of injuries come from not properly training those stabilization muscles. This is one of the biggest problems of using machines for strength, they take the stabilization aspects out.