In the graphics programming I've done as a hobby, if I have a struct that contains a buffer for an image (say 1024px x 1024px = a buffer array of length 1048576), and have functions that operate on that buffer, etc, I either have to:

1) Create a Vec (separate allocation)

2) Make 1024 or 1048576 a global constant that every relevant struct and function references directly (whole program must use the same size at once; forget about exposing it as a library)

With const generics I can make all that code generic over the buffer resolution, meaning I can trivially use multiple different versions within the same build, without doing extra allocations, and I could even expose that code as a library for others to use at whatever resolution they wish