Stone sealing is mainly for jnats, because they have seams and cracks and layering sometimes. Synthetic stones are usually fine, though, as a default, and don't need sealing.
I have heard people talk about sealing synthetic stones that they have issues with Water retention, but I do not recall if it was positively received or not.
Yeah, if you think a stone dries out too fast you can seal it, usually coarser stones, though i haven't bothered because i usually just wash the mud away to clean the surface, which adds more water
For coarse stuff I’ve always had best luck setting up my bridge with a faucet positioned directly over the stone and steadily dripping. Not the cleanest work method, but keeps things from getting clogged up in my experience. Never been inclined to seal a synthetic stone.