You don't "need" them in a literal sense, as the tinting is effective in allowing the A/C to maintain cabin temperature and, as long as you wear sunglasses, the overhead glare in direct sun is manageable, though sometimes annoying.
But, unless you are a big fan of the expansive overhead views, the sunshades keep you further away from the edge of intolerability in heat and glare.
We spent the first year and a half with our Air in south Florida without sunshades and never found the cabin unbearable. But we put them in for a long mid-summer roadtrip, assuming we'd pop them out after the trip. Four years later they're still in place. We don't notice them visually except in the rare instance there's something overhead that we'd want to see more clearly. (You can still see the sun, clouds, and moon through them, though muted.)
So we ordered them for both our Gravities before the cars were delivered, even though we had read that the newer-generation tinting of the Gravity was more effective and that the Gravity's A/C capacity was more robust -- both true. I doubt that we'll ever take them out.
Something I do think you will need is extensions for the sun visors, as they can't block a sun low in the sky.
We find these to be very effective:
