> "Why not just use recursive functions"

One great reason not to use recursive functions for traversing trees is that you can allocate your own stack data structure rather than relying on the call stack itself. In most languages/runtimes, the call stack has a maximum depth which limits the depth of trees you can process, usually on the order of thousands of stack frames.

Managing your own stack usually produces weirder looking code (personally I find "naive" recursive approaches more readable) - but having it as a first-class language feature could solve that!