The niche that immediately came to mind when I saw this was allowing a user to specify a filter expression for some query, either as a simple text-based language or using some sort of UI (with drop-downs and such). Having a well-established spec with existing libraries for turning these kinds of expressions into an AST for transfer, storage, execution later, etc is valuable. I've definitely implemented a simplified, domain-specific version of this kind of thing in various projects over the years.