Bench pressing is a compound exercise primarily used to build overall upper body strength and mass – primarily in the front of the body. It involves the pectorals, front deltoids and triceps. But to a much lesser extent, uses biceps, lats, and the neck muscles as stabilizers.
Like most exercises, there are several variations of the bench press: Flat bench press, performed on a flat bench, incline, in a more upright position, and decline, in a tilted-backward position.
Another variation in bench pressing is the grip width. By varying your grip width, you can bring different muscles more into play during the lift, while minimizing its effect on other muscles. For instance, a wider grip bench press brings the chest more into play, taking away from triceps involvement, whilst a narrower grip tends to hit the triceps more taking away from the chest.
But there is more to grip width than this. Some people benefit far better overall from benching using a closer grip, or wider grip because of their arm length. Those with long arms tend to have a harder time using a close grip bench press, and will probably be better off using a wider grip, which will generate more power and be a much more natural lifting position for them.
Grip width also influences how low or high on your chest you bench, and vice versa. When benching, the idea is to keep your forearms vertical at all times, and it goes without saying that this can become very difficult if not impossible for some lifters the wider or narrower the grip becomes.
Safety and effectiveness, in that order, should always be considered, particularly in an exercise such as the bench press. It would make little sense making the grip so wide the forearms are pointing really far outward, as a means to hit the chest more, when you can simply perform dumbbell flies, or another good chest exercise, for instance.