I use grids all the time and I have to change the grid settings to fit my tasks regularly.
When I do this, the rendered grid updates only when I hit OK, and it makes choosing the step less convenient — if I’m not happy with the resulting grid cell size, I’d have to open Prefs one more, navigate to the section, try again.
Same goes with Constrain Angle — changing it won’t make the grid rotate until OK is hit.
Sure, it’d need a Preview option, to fit the way it works in other dialogs, enabled by default.