They might have originally been invented and perhaps developed as a fun idea, but at some point they were bought for obscene billion$ by Zuckface (2012 for Instagram, What’sApp was founded by former Yahoo! executives, so I think it was not primarily aimed at fun, and was sold to Zuckface in 2014). Zuckface is an evil face-eating mega-corporation whose original purpose was to share objectification of college girls’ pictures, and now wants to eat anything it can, and make already-far-too-rich bastards even richer than they already are, with no limit. i.e. No, the purpose of them is not “so that people can have fun” – that’s just the supposed bait to lure people to make them socially “relevant” and generate “content” and privacy violations so they can rake in obscene amounts of cash (and info-war dominance) for Zuckface. As for why they restrict some aspect to “business” accounts, I don’t know, but I expect it’s part of whatever perverted manipulations they are trying to pull off. They probably are trying to control certain types of traffic, and manipulate who sees what from whom, so they can sell and track advertising and other info-manipulation more effectively.