What Does the Auto Attack Button do?

[Phantasmid]

perhaps it has more relevance when using a game pad, as you cannot right click an enemy to start auto attack?
But yes, if you are using mouse and keys, don't both assigning it at all.

Not relevant for starting your attack at all on gamepad. Hit A/cross while targeting any enemy to start auto-attacking them.

The purpose of the command is to stop auto-attacking without losing your target, if, for some reason, you want/need to do so.