[shiro]

It's a big deal since it's not really about optimization, but more about the overall coding style and thought process.

With TCO, you start seeing functions as composable code blocks rather than something that is called and then return from. State machines are just mutually tail-calling functions---it's not that state machines are written so, but that in your mental model two are the same. And usually a program is humongous multi-layered state machine.

Suppose you want to abstract the part of the loop. You just write a generic version that tail-calls user-passed function. Users can extend the loop freely, even interlacing two independent loops by mutually calling them (which can't be done with callback model without consuming stack frames).

It's really a matter of style, but for those who get used to think in this way, not having guaranteed TCO is like writing code with one hand tied up in the back.