Point? Evolution, innovation. Things don't always stay the same.the square box design was used, and has been used on dozens of turn based games.
Quite possibly due to hardware limitations and computer games being relatively new, at first.it was turn based for a reason
Later, probably to build upon what they already had or copy other games.
Sometimes its just what the developers choose to do.
Grids/hexes are useful for turn based games in many ways. Doesn't mean the two are bound together.
I've seen some very interesting grid based games that were real time, and even a few turn based ones that didn't work on a grid.