Grids are also usefull for lining up multiple connectors. For example to make modules fit on perfboard. They almost got that right by arduino.
A grid also makes it easier to work with arrays and block.
For example in the old fashioned Eprom array “tutorial” I just posted in:
https://forum.kicad.info/t/replicating-layout-features-manually-with-arrays-blocks-tutorial/15332
Heck. It would be neigh impossible to connect a wire to a symbol in Eeschema. (Snap to pin is probably on some roadmap).
While drawing footprints a grid can be handy, but with the Footprint Wizards and pad arrays it does not matter much anymore.
While drawing the PCB itself, Some kind of grid makes for easy alignment of rows of resistors / capacitors and such, but there are so many different (SMD) components with different pitches that grids are useless during routing.
While routing I always trust on the Interactive router to follow the design rules. By setting the design rules (track widht and clearance) you can put more traces on a PCB than with aligning them on some grid. (Unless the grid is an exact match with your design rules.