What is the purpose of a .jpg.out.pp3 file created by RawTherapee?

From the RAWTheraPee 3.0 Documentation:

PP3 files If you have saved the above output file next to your raw, you might notice that there's a third file called DSC_1000.jpg.pp3. This is a little text file that describes exactly what edits you made to your photo. Just open it in an editor to see how it looks like. Whenever you re-open that raw in RawTherapee, this file will be read and all the settings from the last session will be restored (the profile displayed under Postprocessing Profiles will change to 'Last saved'). So you'll never have to wonder what the sharpening settings were for a certain photo that you edited two weeks ago, because everything is recorded in this pp3 profile. (To be more precise: nearly everything, as ranking information is not stored in this file). By default this information is also saved to a cache on your hard disk (here the ranking info is available indeed). In Preferences, tab Image processing, you can disable the generating of these pp3 files. But why would you? These pp3's represent all the work you did on a photo and everybody knows that this can be time-consuming. So keep your work! You might even want to backup these files, together with the processed photos. In case that you want to start a new edit session from scratch, simply select the default or neutral profile again.

In essence it is a commonly known as a "sidecar" file, and saves all of your changes separately.