Most of you have probably heard that it's important to have good arc. But why is it so important? Well, simply put more arc will increase the size of the hoop as the ball goes through.
Take a look at the image below, it shows the size of the hoop when the ball goes through, with different amounts of arc. As you can see with a 30 degree incoming angle (a flat shot), the basket is only 9 inches wide! You're not going make a lot of baskets shooting on a hoop that's just 9" wide. Now, look at the size of the basket on the bottom, with an incoming angle of 90 degrees (maximum arc). The basket is literally twice the size! This is the basket we want to be shooting on.
Let's face it, most players' shots aren't dropping straight through the net like the example at the bottom. However, if you can get your arc consistently up to 50 degrees, your basket is still 78% bigger than a player who shoots a flat, 30 degree shot. Think about the advantage there... just between the top two examples in the diagram below(30º arc and 50º arc), the shooter with 50 degrees of arc will score nearly twice as often as the flat shooter!
How do I know if I have good arc when I'm shooting?
Hold your follow through after every shot and it will give you a lot of information. If your release is not high enough, you will lack arc on your shots. To test this, look at where your shooting elbow is when you hold your follow through. Your elbow should be above your eyes to have optimal arc.
Here's one of my favorite shooters because of his near perfect mechanics. Note the elbow is above his eyes and it'll finish a little higher still when the shot fully releases.