A practical reason you can’t just allow the compiler to do it is if you are doing goofy things like overlaying two structures, void pointer manipulation or cast

A practical reason you can’t just allow the compiler to do it is if you are doing goofy things like overlaying two structures, void pointer manipulation or casting one object to the format of another because you know the first n number of elements line up anyhow, or if you’re changing specific bytes as memory manipulation and not by the object itself. Typically if I define a 8bit as the first element, I need to be certain those 8bits are first even if that wastes three more bytes to align on the next variable.