The NOVA filesystem

The fundamental thing is that emacs communicates to gcc and git by doing one side of a particular set of system calls, and the other ends of this one-to-many-over-time interaction expect to use another set of system calls to retrieve it. Additionally, the system calls have some defined semantics for what the acceptable intermediate states in the source program's sequence to make visible to other programs, including the case where the source program crashes for some reason (not necessarily a power failure).

If you think about the IPC properties of filesystems, it's clear that you need one, even if memory isn't at particular risk from losing power.