Back in the day I had a cool self made theme from Ghost in the Shell, I remember making some very obscure image format from some jpeg I found online. Man what time I have put into customizing my Linux experiences. The computer was truly a (well trimmed) pet. With desktops on kubes with fish inside and reflections etc.
The only time I would want Grub is for that reason primarily. But it makes sense to be very familiar with Grub so you can not be screwed up by systems you encounter which may have been set up, "grub-reinstalled", or configured by those who are not as familiar as they should have been.
Once you do have a regular ordinary multiboot system or SSD that includes one or more Linux installs, with Grub working well, then adding the themes like this is only the icing on the cake.