Why does gravity exist? And how does it work to be good at football?

The question of why gravity exists is not a scientific inquiry, rather it is a religious line of questioning.  Although science can explain the theory of gravity well, it cannot fully explain the entire concept of gravity or its uniqueness in the universe.  Science can basically answer how it works and when it should be expected to occur, but it cannot answer why it exists no more than science can answer why the universe exists.

Gravity is the force of attraction. It is the measurable amount of pull between two objects in the most basic sense. It is theorized that everything with mass has gravitational pull when acting in concert with another object with mass.  This is because the mathematical formula for gravity includes mass as an integral part. The mathematical formula for gravity is:

Fgrav = Gm(1)m(2) / r^2

Using the gravitational constant (G), it is possible to determine the gravitational pull (or force) based upon the mass of two objects. While gravity is understood and widely accepted as a universal force, it does have its own unique properties. For example, the speed of gravity is not fully understood. The theory of relativity suggests a curvature of space-time limiting the speed of gravity to that of light.  However, if operating on theories where such an indent is not accepted, then gravity must travel faster than the speed of light.

Gravity does not help or hurt any part of the game of football (American or European). Gravity is part of the game. However, understanding gravity can help a player improve at any given moment in the game. In additional to understanding gravity, understanding geometry and physics will aide a player. 

Football has a ball with a defined weight parameter to be accepted by the league (supposing it is played professionally). Therefore, the ball will continue to act in the same manner every time unless another parameter changes. Gravity is a constant force on the ball and cannot be changed by the players, therefore limiting the actionable parameters. To best understand let's look at a scenario of kicking the ball. Once the ball is kicked gravity will continue to pull the ball downward. This creates an arc when the force of the kick propelling the ball is overcome by the force of gravity pulling back toward the ground. Even a mediocre player can calculate the landing zone for the kick based on the first half of the arc. The ball will continue on an arcing path downward which will mirror the ascending arc. The only parameter affecting the ball which will greatly affect the landing zone is wind resistance pushing the ball. Mastering the art of arc calculation will help any football player.

Another area where gravity is important is the one-on-one struggle in football. American football has a lot more contact than European football (soccer), but there is still some contact. Because gravity is always working to pull bodies down, people will naturally fall if gravitational pull overcomes our muscular-skeletal ability to hold an upright posture. When two people collide the person who is able to manipulate the other into a position of compromise will win by forcing the other person to fall or slow down to compensate for the gravitational pull on the body.