Question with no game hacking background

I have been programming and creating applications since forever. Hacking, however, is a recent interest of mine to learn. I have been reading a couple of threads and I have seen basic tutorials reading from memory by creating a snapshot of the process. If we have the ability to do this, why do we need to inject a dll?

You do not have to inject a DLL if you are only reading/writing memory. Injecting the cheat into the game process is only so that you can use the game's functions as well as using less processing power, as RPM and WPM are pretty taxing.