Partly out of habit, physicians frequently order lab tests before surgery, to make sure everything is OK. But this is not always useful.
The purpose of pre-operative testing is to figure out what risks there are, and to minimize those risks. If you are going through a low risk surgery, like a cataract, the labs will not provide information that will make you safer, in almost all cases.
"Preoperative testing for low-risk surgical procedures typically does not reclassify the risk estimate established through the history and physical examination, may result in unnecessary delays, lead to downstream risk from additional testing, and add avoidable costs." That's from the Society of General internal Medicine, on the Choosing Wisely website.
Chest x-rays before surgery - any surgery - are also unnecessary, if you have no symptoms. They will provide no useful information in almost all cases. That's from the American College of Physicians.
There's actually a Cochrane review on pre-op testing before cataract surgery. Cochrane reviews are gold-standard evaluations of the evidence respected worldwide. The analysis included three randomized controlled trials, and showed no benefit of the testing.
And yet this stuff happens. One study from Philadelphia found that 53 percent of low-risk patients had inappropriate testing. Usually this was standard tests like a complete blood count and blood chemistries.
Why do these tests continue to be common? It might be due to historical habit. After all, it's hard to go against your training. And it may be fear of liability, because much of medicine occurs due to the fear of a malpractice suit.