What is the input manager for?

I just started programming with Unity(and in general) and was following this tutorial: http://unity3d.com/learn/tutorials/modules/beginner/2d/2d-controllers
and in it he says that although he is using GetKeyDown to get the space bar input it is generally not a good idea and to use the inputmanager instead. So I am wondering how to use the input manager and what exactly it does. Thank you!

As you’ve seen, Unity offers a bunch of functions to help you check for raw input (ie: “Did the user hit the spacebar?”).

If you use the input manager, it’s easier to change keys around, let the user change keys around, or accept some other types of input. Instead of checking a specific key, you let Unity know that spacebar is “shoot” (or whatever) and ask if that input was used (ie: “Did the user hit the ‘shoot’ key?”).