diphenhydramine is the generic name for Benadryl. Occ. useful for systemic allergic reactions. Major side-effect is drowsiness, sleepiness for many, but not all.
It could help blunt a life threatening allergic reaction to say, bee/wasp stings, but if life is threatened the go-to drug is epinephrine injected, followed by diphenhydramine orally or IM.
Many use it help them sleep (Tylenol PM or Advil PM, etc.), but it seems to reduce REM sleep/sleep quality, and increases risk of falling during the night and into the next AM
Keep it around, it's cheap.
Like most drugs (except for nitroglycerin, aspirin, maybe tetracycline and liquid meds) diphenhydramine does not expire anywhere near the "expiration date" the manufacturer has put on the container. The manufacturers can put any date they want on the containers: an earlier date perfectly suits their desire to convince gullible consumers to replenish their stocks decades before their meds actually lose significant potency, so as to reward the company's stockholders and maintain full employment.