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.