You ask a very fundamental question.
The answer: Because choice is everything.
There are several very basic purposes for handloading: Economy, performance, enjoyment, necessity.
One of the Performance goals is to develop ammunition that works BETTER in your guns than factory ammo. One powder may work better for you, you have to test it yourself to find it.
To that end, you have many different powders to try. You are absolutely right to get a load manual like Lymans and study it. Check the components available to you (including powder!), and select a load to try. Start low and work upward, seeking a load that works great in your gun, far better than factory ammo. If you don't find it, change components next time and try again. That's what handloading is all about.
It is very fundamental to handloading to have many choices to choose from.