Comment by jon-wood

I don't think anyone is suggesting to ship a version to actual customers without some polish, but foregoing polish is good advice. I've attempted to make games on several occasions and my current is the first to stick because I intentionally decided not to worry about aesthetics too much at first, that allowed me to quite rapidly get to something that's fun to play, which in turn has motivated me to keep going. It also drastically reduces the pain of sunk costs when the entirety of your projectile system is a coloured box shooting spheres rather than a lovingly crafted gun model which isn't actually fun to use.

Having watched/read a few things about the white boxing stage the general advice is to put in as much polish as you need to do that and no more. If you're trying to prove out jump mechanics literally just some boxes for platforms and a sphere as the character is enough. If you're making a stealth game then you'll need some lighting in your level because it's a core game mechanic.