Hi there
As a vet myself, I can clear things up a bit.
For any prescription only drugs, the animals we are prescribing for must be "under our care".
This is a very vague description, but it is what stands legally.
For small animals with long term conditions, this is usually accepted to be seeing the animal every 3-6 months, and sometimes testing if required for the specific condition.
For large animals, the term is even more vague!
Mostly, large animal vets will expect to have been on the farm some time in the last 6 months to a year, to have an idea of the layout, what animals you have, how they're housed, fed, bedded, etc.
This just makes it easier for us to have an idea what diseases or conditions are more likely, and offer the best treatment.
In a lot of cases, animals can be treated without seeing them, but the best possible diagnosis, treatment and outcome will always come with seeing the animal in its own environment to be thorough.
This is not just a money making exercise, and in general terms, if we see what the farm is like, with an outsider's eyes (I'm not saying vets know better, just sometimes easier to look at the big picture when it's not your own place, as I know with my own smallholding!), we can often help with general preventative care, even general improvements on occasion.
Hope that helps
Suzanne