What Use Is An F-call?

What use is an F-call? This week I'd like to share with you an experience I had on air a couple of days ago. I was tuned to 7.093 and I called CQ DX at about 4pm in the afternoon. I'd been listening on all bands for most of the afternoon and finally was beginning to hear some movement on 40 meters. There had been contests happening on all manner of frequencies, near and far, but for some reason or another, I wasn't getting through. There were a couple of hams having a chin wag a little further up the dial, but they seemed too engrossed to have a chat with someone else, so I dialled up the VK calling frequency and called for CQ DX. The call I got back was: "What are you doing calling CQ DX on this frequency at this time of the day?" - at first I was confused, it sounded like I'd trod on someone's toes, so I asked, "Sorry, is this frequency in use?" - I got an unclear answer and then: "What were you expecting in the way of DX?" to which I replied: "Something outside of Perth