As many others previously, they were created as a gimmick. Back then, most people were used to the good ol' sit-down roller coasters, and to ride a roller coaster where you're in any position other than sitting (in this case standing up) seemed crazy to the people back then, which was why stand-up coasters were so appealing. I know what you mean because I know a lot of people, mostly non-enthusiasts, who are completely confused with why the heck someone would ride a coaster standing up.
Personally I don't mind stand-up coasters. If I came up with the idea I would have scrapped it personally but I don't mind riding stand-up coasters. In fact I really like Mantis at Cedar Point and Shockwave at King's Dominion a lot. I do wish B&M would open a stand-up coaster sometime soon, hopefully with a zero-g roll