I mean, a NAS is literally Network Attached Storage. Your old laptop has storage and, presumably, is on the network; that's a NAS.

The reason why people have standalone NAS boxes is because a laptop usually can't hold all that much in the way of storage. My NAS has 42 TB of addressable storage; that's not really viable on a laptop. Add in any form of redundancy (my 42 TB of storage comes from five hard drives), caching (32 GB of RAM helping with a read cache), or other services and people quickly outgrow a laptop or even a miniPC.

I'm generally of the camp that only have storage and storage-based services on my NAS, so the CPU of my NAS is super weak compared to my actual home server. There is a good chance the CPU in your laptop might be stronger than my NAS's CPU even. Other people combine their NAS with their home server, needing a stronger CPU as a result.

As for why a prebuilt? Some people don't want to delve into that and just want Storage That Works (tm). I don't dive into networking content all that much, hence a prebuilt router instead of something using opnSense or something. I'm happy playing around in the guts of a storage box (it really isn't all that complicated), so I roll my own.

It sounds like a NAS product would be a waste of money for you.

The next step for you is adding more drives, setting up some sort of zfs or RAID array, and setting up automatic remote backups to somewhere like Backblaze.