One reason to use the PC you already have is performance where a NAS will limit you:
While that is plenty fast enough for audio, if your PC contains NAND SSD's and is wired with 10Gb ethernet, you can triple those numbers which may be important for high resolution and high bitrate video files (say Blu-ray rips or your own 2160p video files shot with your high-end camera). Or, when it comes time to back up those large files.
I've been using a Synology NAS for 11 years now and it still has the same HDD's I put in there in 2009 (!). But, it's time to upgrade and I think I'll just house the files on my SSD's and set up Windows for media streaming and retire the NAS. Backups will be done by physically connecting and disconnecting a backup disk so it is not wired into the system 24/7/365. The plan is to house the PC outside the room but that comes with its own challenges such as expensive fiber DisplayPort cables and active USB cables to go beyond the 3 meter limitation. I plan on doing that anyway for noise and heat reasons, but it would be nice to save a little by skipping a new NAS.
