It is common for small CPUs to be embedded inside of various hardware devices. For instance, your GPU might have one or more control CPUs embedded inside. These

It is common for small CPUs to be embedded inside of various hardware devices. For instance, your GPU might have one or more control CPUs embedded inside. These CPUs would have either direct or at least DMA access to main memory. If you have heard about "firmware" being necessary for a hardware device to function, that "firmware" is really just software that runs on one of these auxiliary CPUs. Suppose you want to send a command from the main CPU to this subprocessor. For efficiency and simplicity, that command might be defined as a C struct, in a common header. In that case it can be good to use packed alignment so you don't have to worry about possible layout differences between CPUs.