Wilco is really useful to avoid confusion with responses to requests, as others have said. I have another example:
"Speedbird 6, report ready for departure."
"[mumble] ready, Speedbird 6."
Usually the bit I haven't heard will be 'fully', but if it's difficult to hear, you often have to confirm whether they're reading back the request to report, or actually reporting. In this case, 'Wilco' solves that issue. The other thing that would stop the uncertainty would be not saying 'fully'. After all, you're either ready or you're not - it's not a sliding scale
