While there are use cases for NAS, generally, if you have a desktop PC it's far better to put the hard drives in it rather than setting up a second computer you have to turn on and run too. Putting the storage in the computer where you'll use it means it'll be much faster, much cheaper, incomparably more reliable, with a more natural UI, and it'll use less eletricity than having to run 2 computers.

Now if your NAS use case is streaming media files to multiple devices (TV set top boxes, etc), sure, NAS makes sense if the NAS you build is very low idle power. But if you just need the storage for actual computing it is a waste of time and money.