In many cases, the human body’s immune system will heal itself if you give it some space. While some treatments based around the humors theory (like bloodletting) may seem a bit extreme or weird, most of them would have fallen into the camp of what we today would consider “supportive care”. That is, if they’re too hot (fever), cool them off - maybe with cool cloths. If they’re too cold, warm them up with extra blankets. If they’re too dry, give them water. If they’re too wet (sweating), fan them. As many people wouldn’t have had access to actual physicians, that likely would’ve been the most common application of the principle, and it would make sense for many illnesses. It doesn’t really help anything (it doesn’t kill bacteria or viruses), but the idea is to support the body and let the immune system do the work.

You mentioned bloodletting, which sounds utterly barbaric to us today. But in correct context, it wasn’t always a bad or dangerous medical practice. Leeches have a chemical in their saliva that helps to prevent blood clots, so letting them feast on injured tissue near a nasty bruise or burn (for example) can help prevent dangerous clots and tissue damage. Maggots are also used in modern medical practice to clean dead skin off messy wounds and prevent infection.

But many of the treatments used back then would be familiar to herbalists and naturopaths of today. Most issues would have been treated with a change in diet or an herbal tea. More extreme measures like bleeding a patient or using hot cups to “relieve an excess of humors” was often used for those cases where the patient was likely to die in any event. This wasn’t universal, but common usage. After all, there was no American Medical Association or professional body governing who could be a physician or who could teach physicians! But even when Galen first posed his theories, he posited that digestion and the digestive processes were responsible for the balance of humors. Paracelsus in the 1500s was promoting herbal or what then passed for pharmaceutical treatments for disease, but it wasn’t really until the 15th century that the mechanical methods came into use. There is some dispute to this day as to whether they ever gained widespread popularity or were simply popularized by playwrights and satirists of the day; it may have been only a handful of nut jobs promoting the treatment, like the Snake Oil of its time. We do know that there were several prominent scientists and physicians in that era that did not approve of those mechanical means of humor balancing.