Reaper projects are normally designed to render out a single audio file, or minor variations on a single audio file, not multiple unrelated audio files. This is how Reaper is designed, and probably feels most natural to Reaper users, so we encourage Reaper users to continue using Reaper this way. So, to answer your question: It’s usually best to use one .rpp file per sound effect (or per event, if you’re using the option to import a reaper project as an event, as I describe below).
The main benefit of the reaper integration is that it allows you to open an asset’s .rpp file in Reaper from FMOD Studio’s assets browser, allowing you to quickly iterate on an asset without having to manually locate its .rpp file through your operating system or Reaper’s file picker. FMOD Studio automatically reloads the asset after such an editing session. If you are not using this feature (either because you rarely iterate on an asset after importing it into your FMOD Studio project or because you prefer a different workflow for iterating on the .rpp files of existing assets), then you are correct that there is no benefit to importing the .rpp file as an asset.
The other benefit of using Reaper is that it is possible to import a Reaper project as an event, rather than as a single asset. This means you can do a great portion of an event’s design in Reaper rather than in FMOD Studio’s event editor, if you prefer. Again, if you prefer not to use this feature, you will not benefit from it.
Simply setting your Reaper project to render out to your FMOD Studio project’s assets directory is an entirely valid way to work, and involves very little hassle to set up, so there’s nothing wrong with working that way if you prefer.