Does a process’s parent have any significance from the perspective of its child?

When I saw this question, I was pretty interested because I know I've seen getppid used before..but I couldn't remember where. So, I turned to one of the projects that I figured has probably used every Linux syscall and then some: systemd. One GitHub search later, and I found two uses that portray some more general use cases (there are a few other uses as well, but they're more specific to systemd):

• In sd-notify. For some context: systemd needs to know when a service has started so it can proceed to start any that depend on it. This is normally done from a C program via the sd_notify API, which is a way for daemons to tell systemd their status.

  Of course, if you're using a shell script as a service...calling C functions isn't exactly doable. Therefore, systemd comes with the systemd-notify command, which is a small wrapper over the sd_notify API. One problem: systemd also needs to know the PID that is sending the message. For systemd-notify, this would be its own PID, which would be a short-lived process ID that immediately goes away. Not useful.

  You probably already know where I'm headed: getppid is used by systemd-notify to grab the parent process's PID, since that's usually the actual service process. In short, getppid can be used by a short-lived CLI application to send a message on behalf of the parent process.

  Once I found this, another unix tool that might use getppid like this came to mind: polkit, which is a process authentication framework used to gate stuff like sending D-Bus messages or running privileged applications. (At minimum, I'd guess you've seen the GUI password prompts that are displayed by polkit's auth agents.) polkit includes an executable named pkexec that can be used a bit like sudo, except now polkit is used for authorization. Now, polkit needs to know the PID of the process asking for authorization...yeah you get the idea, pkexec uses getppid to find that.

  (While looking at that, I also found out that polkit's TTY auth agent uses it too.)

• This one's a bit less interesting but still notable: getppid is used to emulate PR_SET_PDEATHSIG if the parent had died by the time that flag was set. (The flag is just a way for a child to be automatically sent a signal like SIGKILL if the parent dies.)