90% of beach fisherman are not hardcore, jumping around the jetties in a 25mph NE blow, kind of people. Most folks would just as soon put a chunk of bait on a hook, toss it out, set the rod in a sand spike and watch the rod tip.
A baitrunner makes that easier. How many times have you seen guys with regular spinners get a bite and wind up chasing their rod and reel into the wash because they didn't back off the drag?
Or, if they do back it off and then get a bite it is much harder on a regular, front drag spinner to reset the drag back to the correct tension.
I'm not saying a baitrunner is the
only way to go. Personally, I've never owned one. But, I've sold a ton of them over the years and had a lot of positive feedback on them.
BTW, when spinning reels first came out, many people called them gimmicks too.