The applications are quite fascinating. A good place to start https://en.m.wikipedia.org/wiki/Applications_of_the_Stirling_engine

Just to tag along on that last sentence, Stirling engines do have a couple niche use cases today such as cryocooling. Instead of using a temperature differential to create mechanical movement, one can apply mechanical movement to create a temperature differential. In commercial applications these can reach down to 40-50 Kelvin. I just think they're neat.