It's also useful for signaling between processes to have them continue doing something as long as this proc path exists (the fd can also just be backed with >/dev/null instead of a real file that needs to be removed)
Normally, the pid and fd could be reused, so you should either make it a known unique path and verify it's still the same, lock the pid via some means (not sure if and how it's possible), or use another technique.