The game mechanic is the part of a game that separates it from a book, a movie, a photograph or even a conversation.
All told, if a designer has to pick one or the other, more games can find success with a weak theme and a strong mechanical core. Whereas games with an incredibly cool theme but weak or boring mechanics very rarely stick around for long.
Jumping between mechanic and thematic lenses helps keep the game’s design process fresh.
The reality is that a mechanism without a theme is still a game, but a theme without a mechanism is just a theme.