Why do I care about a 370-year-old medical text? Surely it's irrelevant and outdated?
Yes, it's outdated (as a medical text); no, it's not irrelevant!
Please, please, please do not take anything in this book as actual medical advice! While many of these plants are still used in modern medical herbalism, we're much more aware of safety, impacts, and interactions now, so if this is what you're looking for, there are other books on this topic, written by modern experts, who have an understanding of how to use herbs safely - and please consult with a suitably-qualified expert before starting any new medical treatments or regimens.
However...
Are you interested in plants and botanical history?
Culpeper's Herbal is a marvellous reference of plants common in the London area in the 17th century, their names, and the beliefs surrounding them (plus you'll love the illustrations)!
Are you interested in the history of medicine or medical herbalism?
This book was used as a genuine medical resource until the advent of modern medicine almost 200 years later!
Are you interested in herb-lore or magical herbalism?
Culpeper was one of the leading medical astrologers (to the extent that he was actually accused of witchcraft in an attempt to discredit him), with each herb profile including astrological associations, temperaments, their actions within Hippocrates' humoural medical system, and a bunch of other beliefs and folklore mixed in. A huge portion of modern magical associations of plants derive directly from their histories of medical uses.