Since CSS grids are mostly used for the global page layout, the ideal use case is using it to design complex pages that would be very annoying to make with other techniques, and on older browsers fall back to a much simpler design with flexbox or floats.

Since those fallbacks only require a handful of lines of CSS, it only takes a few minutes to get the compatibility to almost-perfect, and you can get a better design in front of most of your users very quickly. To me at least it's a solid mix of design freedom and development speed.