The only reason to follow the standardized sizes is if you want to sell out to a big company down the road, or use pre-printed vinyls, which is all some advertisers want to use like McDonalds (they are printed in standard sizes by McDonald's Corporation, and sent out to the franchisees).
Otherwise, you are correct that it is meaningless.
The reason the 30-sheet poster is the size it is is becuase that was the most efficient size that they could paste "wallpaper" ads on in about 1950. Before that, it was called the 24-sheet and was slightly smaller. Of course, they don't "paste" that many ads on anymore, so it doesn't much matter. But since that's the official size, and has 1,000s of faces in their inventory, that's what they stick with.
If you go to sell your signs someday, a 14' x 48' will get a much larger price than a 14' x 42' -- simply because it is standard. Until you go to sell, however, and assuming the advertiser doesn't care, it doesn't really matter on a day-to-day basis.