Because sometimes, you want more directions than just 8 directions. This optional setting will turn your mouse cursor into a red pointer, which your character will follow. The aiming reticle will be controlled with your keyboard keys, which I believe most people has in WASD.
The character speed changes based on how far your cursor is to your character. If it is within 1.5 tiles to your character, you will not move at all. The further your cursor is to your character, the closer you will move to your max speed. If your cursor is 4 tiles away or more, you will move at 100% speed, with everything in between of 1.5-4 tiles as a gradient form 50%-100%.
Also, if your cursor is 4 tiles or more, its sprite changes to a double-pointer/fast forward symbol. Your cursor rotates based on its location to your character. Its pointy end is always pointing away from you.
Alright, please bring your reasons to why you will or will not use this setting!
Personally, I would use this setting to navigate through bosses attacks, especially in tombs, or shatters.