Data redundancy. I have two mirrored 2TB disks, so one can fail without data loss. Advanced filesystem (ZFS) nullifies silent data corruption and provides automatic integrity checks, multiple copies, compression, encryption, deduplication, etc. There's 10 years of my personal files (work) currently stored there.
Got an Atom embedded mainboard cheap (D510, 2x1.6GHz, 4GB RAM - 85e) four years ago, and took one 1U rack case with power supply from my ex company. Completely fanless system with low power consumption that can run 24/7 without making noises. I need development machine that's going to be online, so this was a necessity. When HDD prices fell down a bit after 2011, I purchased those two drives. Hence the file server.
...and I can access all data from any device at home (cellphone, laptop, media player, Wii).