I have to use it every flight as its part of our pre-flight checks
You have a Rolex watch don't you? Every pilot that I flew with that had a Rolex watch checked their watch with the HF, frequency usually 15.000, every time they got into the cockpit and then reset their Rolex to the correct time. My Seiko Mickey Mouse watch never lost a second.
The answer to the question asked is, one uses HF when ATC tells you. The only time I was taken by surprise that I had to use HF, was over India the first time I flew over the country. VHF coasting in, but over the interior it was HF.