Using Private Vehicles for Work: Insurance Concerns and Restrictions

Hello, I started a job at a car registration office, mainly driving car dealerships there. The company has its own vehicles that I can/should use. In any case, they are all small, old switching cars, without navigation or other equipment that makes work easier.
I would like to use my private vehicle, which has navigation etc., because I can work and handle it better.

But the company told me that they didn’t want me to use my private car for insurance reasons?
My vehicle is fully comprehensively insured and I am responsible for it, and I am the only one driving it. I don’t really understand, can I now be banned from using my own vehicle for the tours? What is the difference between having an accident in my own car on my way to Rewe or driving around for company registrations and then having an accident?
It bothers me a bit because the vehicles don’t have a navigation system and there are switches and I have to have my cell phone on my lap every time I drive.