The Scriptorium increases your total levied silver by 0.2% per level. For exmaple, if you level up this building 5 times, you will recieve 1% more silver from each levy. The Scriptorium has no other purpose; but silver is always in high demand. So if you can spare the build time, leveling up the Scriptorium is worth its weight in silver.
The scriptorium becomes available at senatus level 25.