What made PWD, Daman & Diu hire private vehicles?

There are some vehicles plying on roads of Daman with a banner of “ON DUTY PWD DAMAN”.

There are many vehicles owned by PWD, Daman & Diu like trucks, cars, motorcycles, etc. then why they need to hire more vehicles?

Moreover, under which contracts these vehicles are hired? For which works or officers, they are hired?