1. You own it
An on-premis (on-prem) system is yours. You own and manage it. You're not dependent on anyone else to get it to work, which of course means you are responsible for making it work. But if you don’t want to do that, you can hire out some parts of the maintenance to a vendor. Just be sure to pick one that is trained and certified on your specific system.
2. It's highly customizable and robust
Highly customizable, an on-prem system has a long shelf life--typically, 10 – 20 years if run in a proper environment (not to hot, not to dirty). Overall, on-prem systems offer every day performance and relatively easy maintenance.