The FSX source code has been licensed to Lockheed Martin, which they now resell as Prepar3D for "non entertainment" use only. It's supposed to be better optimised for modern computers and compatible with all the add-ons.
As for why you would bother when it doesn't contribute to your currency, well you can practice navigation with and without failures to your heart's content in the comfort of your own home. To reach the same confidence with navigating on raw NDB or VOR data practicing in a real aircraft or certified simulator, would cost you a lot of money.
I find it a useful complement for practicing instrument approaches before attempting the real thing, where you may have other traffic and ATC requests and proper IMC to contend with.
Not to mention the computer can also be used to do other things like post on PPRuNE
