Translation box in VS of GTM. Why we need that?

The key is this sentence in SOL14707: "The metrics retuned from an iQuery probe are based on the private network IP address configuration of the probed device."

 

So even though the GTM BIG-IP monitor (iQuery) is directed at the public address all of the metrics which LTM reports to the GTM contain the private LTM addresses. Since GTM knows about those private address due to it's "translation" field, the metrics can be matched up correctly with the public address in GTMs "address" field.