Idk about merlin, but with iNaturalist, the purpose was logging sightings and locations of plants/animals/fungi/etc, and letting people double check your ID so it can be used in research.

This built a massive database of images, locations, times, etc, which is a really good training set for image (or sound) recognition. Now the app is really good at automatically recognizing stuff from pictures, so people just use it for that. The app actually started to try to penalize you for using it for identification without actually logging stuff.