I think nearly every active new user on iNaturalist does this, which suggests to me either that it is somehow a natural response or that the system somehow encourages it.
As others mentioned, I think it’s a combination of acknowledging the ID (and assuming the identifier is an expert), wanting to get RG, and looking at the species photo and thinking “that looks about right”. Yeah?
However it happens, I am confident that it has resulted in enough observations of difficult-to-ID species becoming Research Grade that they get the species into the machine learning suggestions, which people then start using to identify their own observations.