As the name would imply, Quick Cast is a meant to be a faster, more efficient way to use your abilities. I say meant to be, because it can be difficult to use properly if you are not accustomed to it.
Quick Cast is quite common in other MOBAs, but since many Heroes players come from other Blizzard games they are often more used to the traditional point and click method of targeting abilities. If that is what you are used to, it may take some time to get comfortable with Quick Cast, but the benefit is worth the effort.
There are three options for Quick Cast.
The first is Off, which is the default setting. This is the standard click to target method, and while it is the easiest to use, it is also the least efficient as it requires a keystroke to enter targeting mode and then a mouse click to trigger the ability. That is two actions every time you wish to use an ability. This option is for newer players, or players who do not have previous experience using Quick Cast, or who simply don’t like it.
The next option is On Release, which will cast the selected ability at your cursor location when you release the ability key. It is slightly faster than Off, since it only requires one action to use an ability, and you still have your targeting option until you release the ability key. This option is ideal for players who are ready try using Quick Cast, but who still want to retain the targeting display. This option can significantly improve your efficiency, and it is only slightly more difficult to use than Off.
The final option is On. This is the most difficult setting, but it is also the fastest. When this option is enabled, pressing an ability key will immediately cast that ability at the location of your cursor. Since the ability is cast instantly, you don’t get any of the normal targeting displays that you do with Off or On Release. This setting is intended for advanced players, or players who familiar with Quick Cast from other games.
You can access Quick Cast from the Mouse and Keyboard tab on the Main Menu.
The best way to test out the different options is to take a Hero like Jaina or Valla, who has several targeted abilities, into a Training game. Switch between the different options and see how they affect using your abilities. Once you understand how each mode works, you can decide what feels right to you and test it out with some AI games.
It is important to remember that there is no ‘best’ option. Don’t let anyone tell you that only noobs use Off, and that all of the good players use On. It is all about what feels comfortable to you. This is a tool that can help your game though, so you should at least experiment with the different options before deciding whether Quick Cast is right for you.