Nothing, if you also stay a day or two along the western coast (but if you want to save money, it doesn't have to be at the FS Marriott, though in that case it will have to be some other brand).

The Courtyard is a better location for doing thing other than lounging on the beach. Going up to the volcano mountain (to be there at sunset or to arrive before sunrise). It's near the needle mountain park. It's much closer to the Hana drive; in fact, there was much to see on that drive, I took it one day part way and then back, and then a second day "zooming" past what I'd seen before completing the circuit. There's no way I could have that from the FS Marriott, it's in the wrong location for that.

Also, there are restaurants nearby the Courtyard, and since the parking is free you can do dine anywhere, including at non-exorbitant rates. Good luck with that a resort.

But while you want to do most of your stay there, if you have enough days you may want to do or two nights along the western coast, it's just a different experience. (But it's one experience, while from the Courtyard you have better access to many experiences.)