Optimization and functional recursion is nice and all, but the unsung killer-feature of (mandatory) TCO is to (ab)use the stack as a state-machine. Each "state"

Optimization and functional recursion is nice and all, but the unsung killer-feature of (mandatory) TCO is to (ab)use the stack as a state-machine. Each "state" is a function which tail-calls the next state, so it replaces the stack frame. Stack-traces now double as state-inspectors. Substates naturally follow from subroutines. I regularly use this in Lua gameplay scripting, which has proper tail-calls in the language spec, not just as an implementation-optional detail.