You could totally turn on as needed, WakeOnLan is good for that. But typically when people run a NAS it is for streaming audio, video, file sync and backups and maybe docker running other services so the NAS is typically on 24/7 so it is available on demand. But it doean’t have to be 100% uptime if you don’t want it to be. For example I have two OpenMediaVaults one on a pi and one an old IomegaNAS. The pi is on always with an attached drive, and serves Samba Shares and DLNA/DAAP shares. Has docker running syncthing, CUPS print server, Trillium Notes, and homeassistant; so makes sense for it to be on all day, especially because my wife’s system backsup to it daily automatically. The converted Iomega NAS is mainly a backup machine sInce it is old and not as performant (only has 100 network speed. So that gets turned on to do a bulk backup and not much else.