During the blood test, the blood is screened for clotting, bleeding/blood count, blood chemistry, liver function, and HIV. Generally the test is to determine your general state of health and whether you are fit and well enough to go through with the surgery.
A responsible clinic should ask you for your medical history during the early consultation stage.
If you have any doubts about your general health then it might be wise to have a blood test with your GP prior to any surgery just for your own piece of mind to ensure that you have no abnormal levels that might postpone the surgery......just a thought.
It is good medical practice and safe guards both patient and clinic to see the blood condition prior to surgery. Imagine if not done and after a patient says the clinic gave them a condition they never had prior and there was no prior blood test to substantiate anything. But aside from that is helps in preparation for surgery and if a patient is HIV positive then there would be more checks and safeguards put in place prior to and during surgery. So it is a win for both to do this and it as said here is a test for several conditions that can affect surgery.