First: Do you have backups of your single high-capacity HDD? That's my biggest worry. What's your plan for when that HDD fails one day, as it will?
The main reasons people go for ZFS and a "NAS" is checksums and data integrity protection, as well as maybe not wanting to keep their primary machine awake all the time (I personally don't).
Then there are useful features like snapshots, which means I don't have to worry as much about accidentally deleting, or over-writing a file and losing data.
I don't see anything wrong with using a main machine that's up 7x24 as a NAS, don't buy things for the sake of something, but I'm worried about your reliability and bitrot protection. (Yes, it happens, I've seen it first-hand thanks to ZFS).
The main reasons people go for ZFS and a "NAS" is checksums and data integrity protection, as well as maybe not wanting to keep their primary machine awake all the time (I personally don't).
Then there are useful features like snapshots, which means I don't have to worry as much about accidentally deleting, or over-writing a file and losing data.
I don't see anything wrong with using a main machine that's up 7x24 as a NAS, don't buy things for the sake of something, but I'm worried about your reliability and bitrot protection. (Yes, it happens, I've seen it first-hand thanks to ZFS).