This strikes me as the most pointless excuse for an app - if you are technically inclined enough to understand why using Cloudflare’s DNS in place of your cellphone service provider’s could be beneficial, you are probably also very much capable of typing “1.1.1.1” in the network preferences on your phone...

EDIT: I stand very much corrected, at least with regard to iOS and mobile carriers - I wrongly assumed DNS settings were exposed for the mobile connection the same way it is for WiFi, where it can be very easily manually overridden. As someone who doesn’t use an Android phone I’m even more surprised from comments below that Android doesn’t even allow this for WiFi via the stock settings app.

That this would also allow you to set cloudflare’s DNS globally for all WiFi connections on iOS rather than the current Settings app’s per-network basis is also an interesting advantage.