For me it was simple, although I was an IT manager and not a salesman, I was logging enough miles visiting all our remote branches a few times per year that the cost of paying mileage was more than buying a vehicle for me. So I got a company car. That ended when we sold our company though... Of course, this was a family business, and being family had perks.
Frankly I think every single business is going to be unique in how it determines if you need a company vehicle. Beware though, recording mileage, tracking personal vs work use, and dealing with expense reports for fuel is kind of a pain in the ass. And it isn't worth having unless you can use it as your primary vehicle and not also have a personal vehicle, so you end up being restricted with your only vehicle. When it is all said and done, the money it saved me was only marginally worth the hassle. It stings a little to pay for full tanks of gas after over a decade of not having to foot that bill, and insurance is a bummer, but the freedom of being able to use my own car for whatever I want and not have to record every mile is kind of nice...