In technical drawings (but not only), sometimes we temporarily need a finer grid (more subdivisions) just for a couple of work steps, after which we return to the original (larger) grid.
It is painful that for this we always have to go and change Illustrator Preferences (back and forth).

Feature suggestion:
In Illustrator Preferences make it possible to define two different grid subdivision values: "default" and "alternate".
The "alternate" grid is such that it becomes active immediately when holding a specific keyboard combination (e.g. ALT key) while drawing.
As soon as the keyboard key combination is released, the grid returns to the "default" number of subdivisions.