Smaller launch to a specific orbit. As an analogy, freight trains are one of the most efficient, cheapest ways to move freight. But moving my laundry to the laundromat is better served by another vehicle.
Yeah, that's the main proposition Starship has for the market. Going to analogy with laundromat, you need to include the cost of single-use laundry cart and remember that your laundromat is on another end of the same asphalted road which the truck nearby can easily drive on.
Not sure at all if costs - and hassles, of course - of buying a cart are less than some change for the costs of the fuel - the truck is autonomous, of course.