Setting a process as the reaper means descendent processes are reparented to the reaper, which has to call wait like pid 1 does in order to collect the process'

Setting a process as the reaper means descendent processes are reparented to the reaper, which has to call wait like pid 1 does in order to collect the process's info and remove the zombie. This changes the default behavior where they are reparented to pid 1. You don't set this on pid 1 because, as you point out, pid 1 is already the reaper of last resort. You don't set this on the service process either; it is set on a process manager that isn't pid 1, so that process can monitor and collect info about its subtree of processes.