Chair seat cushions have a reputation for being the thing you buy to fix a chair you actually don't like, and most of the time that reputation is earned. A few extra inches of foam won't save a dining chair with a bad seat angle. But for a chair that's basically fine and just hard on the back of your thighs after twenty minutes, a good cushion is worth every bit of the fuss.
Check a few things before you commit. Squeeze the foam through the cover, because a lot of these are a thin layer over something closer to cardboard than cushioning, and you'll know the difference inside a week. Look at how the ties or straps attach to the chair, since a cushion that slides around the second you sit down is worse than no cushion at all, it just becomes one more thing you're adjusting during dinner. When it arrives, check the corners sit square against the chair back and that the piping, if there is any, isn't already pulling away at the seams. That's a manufacturing tell, not a wear issue, and it only gets worse.
These make the most sense on dining chairs and kitchen chairs, the ones you actually sit in for a stretch, and on any wooden or wicker chair that looks great and feels like punishment after the first course. Breakfast nooks, home offices with a hard wood chair that didn't come with a cushion in the first place, a reading chair by a window in winter. Less essential on upholstered armchairs that are already doing the work themselves.
Over time the foam will compress unevenly if you always sit the same way, which, let's be honest, you will, so flip and rotate them every month or two to even it out. Covers with removable zips are worth paying a little more for, since you can actually wash them instead of spot cleaning around a wine spill and pretending it worked. Ties fray before foam dies, in my experience, so check those first when one starts to feel off.
They sit easily with a plain wood table, a woven placemat, a linen napkin, nothing matched too precisely. A set in a single solid color across mismatched chairs does more to pull a room together than people expect, and costs a lot less than replacing the chairs.
First room I'd put these in is the kitchen, on whatever chairs get used every single day, because that's where the extra ten minutes of comfort actually gets noticed.