Theres a difference between having value to users because its a decent app and they get to use it for free, and generating revenue from those users. Ask those users to pay and they will leave in droves.
Which is a problem for advertisers. not to be meta, but its true. marketers want to but mindshare, and they want to make sure their pitch is not going to be taken out of context. not knowing what your pitch will look like in context is usually a non-starter for ad sales guys. Of course SnapChat may not want to deal with advertisers as that would require tracking and verification, which might put off some end users as well. That seems to leave fewer possible business models. Although they may invent something new, so lets not count them fully out.