There’s a few reasons why that can be relevant.

In the US, a lot of dentists use nitrous oxide for pain management and/or sedation. Taking a blood pressure is one of the safety precautions for pretty much any sort of sedation. In Europe we usually use local anaesthetic; it’s less invasive.

There are also certain complications that can arise from a high BP. It can result in more bleeding, which is not ideal especially if the patient is taking medication like blood thinners.

And on a general note: a lot of people don’t know they have a high blood pressure. It’s a good thing to check occasionally if you don’t. High BP can have a wide range of health concerns.

Someone else commented before me and had a very accurate answer!

In short, blood pressure can mess with anesthetic, making it not work or work less effectively. My dentist wants to make sure they are not torturing their patients with unneeded pain.