There are several reasons I've evolved my system to use a revision control system.
1. First and foremost, not all files in my repository are write once. Many are, but not all (think resume.doc). Plus I store my active coding projects in the repository.
2. Easy replication. Makes it easy to have copies of each file in every client. Something like a RAID or backup system doesn't help here. (The job for a RAID is to host the Perforce server.)
3. I want something that works across operating systems and across decades. Revision control is well understood, reliable, prevalent, and here for the long term.