A new, though very common, service among the car rentals is the transfer. The transfer is usually defined as a one-way or two-way passenger and luggage transport by a rental car with a driver to any given destination. Now you might say - this definitely sounds like a taxi and you won't be off by much. There are of course some slight but important differences.
First off there is the way of organizing the trip with a transfer hire car. You can make a rental car transfer reservation months before the trip for a specific place and time whereas a taxi you have to find on the spot. This is a big advantage especially when you travel in a foreign country with a plane for example and you don't have the slightest idea how to get to your hotel. And the hotel might not even be in the city next to the airport.
A second advantage of the transfer hire car is the fixed prenegotiated price, not influenced by route choice, traffic conditions, etc., as for the taxi, these can definitely add up to your bill.
Another advantage of the transfer is the possibility to rent a car with a driver only for as long as you actually need to. Let me give you an example. You arrive in Bulgaria at the Sofia airport on route to the seaside or to the mountains, where you intend to spend the next two weeks resting on the beach or skiing down the slopes. Why do you need to hire a rental car for the full two weeks, when you intend to drive it only on the arrival and departure dates and leave for the duration of your vacation parked on the parking lot of the hotel. Now you have the much cheaper rent a car transfer as an alternative without giving up the comfort of your own rented ride.