You have better control over the costs of repair parts and maintenance supplies.
There is no salesman to worry you about unnecessary repairs or modifications.
Your personnel become expert with your own equipment. Your costs go down while your standby power reliability goes up.
If you use an outside company like the equipment distributor or dealer, you should realize that they have a financial interest in your equipment needing repair.
It may not be the best idea to have the company that does your maintenance also provide repair service.