Many of us, we use the Raspberrypi because of its low power requirements.
For a home server, running 5 watts when the drives are asleep is great.
Like running a night light.
Rich Prim
[Moderator edit] Attention: If you arrived here because you read 'Raspberry Pi is a slow NAS'... things have changed with the RPi 4 which is the first RPi variant suitable for a NAS due to real Gigabit Ethernet and USB3.