Despite the popular belief that veterinarians scam people for money (I have heard it many times) most doctors simply want to practice good medicine on the pets.
The blood test is need for any unvaccinated outside cat with outside parents(even inside cats need the test in many situations).
I have had cats die during or after surgery to discover that the cat had a disease that I did not find because the clients would not allow any testing to be done.
But of course the veterinarians still get the blame.
So, the test was needed pre vaccination or surgery.
The diseases they tested for is deadly.