Other than keeping your computer on all the time. What is the difference between using a NAS and just putting a NAS drive in your computer for the media?
A NAS is purpose built for the job, very stable, very low energy compared to a PC. A good modern 2 bay NAS will idle at 3w to 6w and when in use (being read etc) rarely exceed 20w contrast this with a modern PC.
NAS are designed to offer RAID security, which is more tricky to setup and manage on a PC, though obviously doable. The NAS are just specifically designed to do this, which a PC is not, so perhaps offers a little more security, I say perhaps. They are certainly easier to use for a more novice user. if 1 drive fails, you don't lose your data, basically, which you would with a single drive solution.
You can also locate the NAS anywhere with a LAN connection. Mine is in a cupboard, silent, out of sight and out of mind.
A modern NAS (talking Synology, QNAP etc. (not the WD rubbish), can also be accessed from anywhere safely, inside or outside if you enable it, and by virtually any device, phone, tablet, TV etc.
Not all NAS can transcode media, but most or all can run PLEX. That depends on the processor.
The software environment is key for a NAS, generally Synology is considered the best software, with many 'apps' that can be installed and used. for example, cloud backup, share sync between NAS devices etc.
QNAP generally offers more hardware for the money, and a good software package, but not as good as Synology. Synology hardware is generally a little more expensive. Pick your poison.
Personally I have 2 Synology NAS, at 2 different sites. The mirror live to each other, and both run and Transcode PLEX, plus backup my cloud files etc. I prefer to invest up front in the hardware I need, and then use the better Synology environment to achieve what I want, need easily.
They offer my access to my files from anywhere, security, and backup when combined together, plus a failover. I have replaced my PC's a number of times in the last few years, but with the NAS solution, dont have to worry about my data security being reliant on my PC.
It is not a perfect system, I am aware of that, but a NAS offers increased peace of mind, data access flexibility and security and cost savings re. elec.