As mentioned before, mast base pressure is a big plus. More importantly, I find I can be more consistent with the mast base pressure on windy, gusty days where the pressure on the sail varies. Without the harness, I get jerked around more and it's more difficult to keep steady flight.
Going hard upwind is more difficult without a harness as well. It requires a bit more muscle and you being out to really grind it upwind. I have a friend much like you who gets by mostly without a harness. We tried a race together and he found going upwind in a race a whole different game than what he thought going upwind was. He was already disadvantaged not having racier gear but he struggled to stay sheeted in all the way to drive upwind even remotely like everyone else was. (I'm not saying freeride gear won't go upwind but if everyone else is making 5-10 degrees higher than you, it becomes obvious pretty quickly.)
You don't have to have a harness which is what makes foiling such a great gateway from non-planing to planing/foiling conditions. But it will give you more flexibility to handle varying wind and explore more upwind and downwind. Try it a bit and see how it goes. No requirement to stay hooked in all the time.
Edit: Having a harness means it's easier to wave at people as you glide by and for photos.