I've used Duolingo and like it for learning grammar and vocabulary. Why would someone pay for it though?

I would have been prepared to pay but I don't see why I would. The only paid features I see are to remove ads (which you rarely see compared to how long you use the app for), offline use (which isn't an issue if you have good mobile internet) and to skip to harder stages (which seems counterproductive).

In terms of making an income, it feels they give far too much away for free. Do they make most of their money from the ads? Or so many people use it for free that it's okay if only a low percent convert?