I have a very obscure allergy to some specific compounds in some medication. It gives me a rash, nothing serious. I'm always very diligent in mentioning what little I still remember about that allergy.
Those conversations don't seem tomatter much, because when getting common small procedures done (such as mole removal) the doctor will use whatever they prefer. Epinephrine is a vascoconstrictor, so it helps with bleeding and keeps the anesthesia (eg: lidocaine) from wearing off as quickly. So, doctor's prefer it. So each time I have to ask the doctor if that's the variant they are using, and if they could do without. Sometimes they do, sometimes they explain why it's best not to and we go ahead with.
I know I could outright say allergy and they wouldn't use it, but I really don't want to cut off a useful tool for them, for no good reason other than I don't like a minor, short-lasting side effect.
Obscure allergic reaction and obscure disease often look the same. You treat them opposite. The former, you want to suppress the immune system, and the latter, you don't. In obscure circumstances (e.g. a doctor is debugging a serious rash after a surgery while you're unconscious and on an IV), those sorts of tidbits can be important.