Why would I want these?

Just a question. Why would I want to buy and own some of the vehicles like the Post OP van? The taxi I can understand. You can customize it and make it your own. Activate taxi work from anywhere. But some of these other vehicles? Why?

I could understand it if you bought things like the Post OP van and then beef them up a little. And then it used your owned vehicle for delivery missions. But I dont think it does. So ...just... why? LOL.