Why some vets perform pre-surgery blood tests on dogs?

Hi, I wonder Why some vets do pre-surgery blood work on dogs, is it really needed or its not necessary ?

for example my dog had a tail surgery and my veterinarian performed some pre-surgery blood tests on him ...

thanks

Many veterinarians offer pre-anesthetic screening to their patients. Thats because it provides a way to assess kidney and liver function prior to undergoing anesthesia among other things. The liver and kidneys are the primary routes that the anesthetics are broken down and removed from the body. If they aren't working well, then anesthesia may be more of a risk. There are many anesthetic agents available, and your veterinarian may also use the blood screening information to determine the best anesthetic protocol for your pet.

Thank you for the fast reply ...

Before your dog goes under anesthesia for surgery or dental work, your veterinarian will require blood work. Why do we ask for blood work? ... There are several reasons for this test, all with one goal in mind -- making sure that your dog is healthy enough for anesthesia and surgery.