Protecting The Family Jewels
I played Little League Baseball for about 10 years growing up, and generally wore a cup for most of it. I played a lot of catcher in my later years, and you only make the mistake of not wearing it once. But the dang things were always stupidly uncomfortable, which is why I wish they had the Nutty Buddy when I was a young lad. Be sure to watch the demonstration video where the guy takes a shot with the baseball coming from a ball machine at nearly point-blank range.
Also of note: While women are vain with clothes sizes (which is why a size 8 isn't like it used to be), there is no "small" size for this cup. No man wants to wear a small cup and admit to it.