But you can just change it in the settings without the app and IOS doesn't allow multiple VPN connections so if you use a normal VPN you can't use this but you could configure it to use 184.108.40.206 anyways
Android (pre-P) made it really difficult to change DNS settings - you have to disable DHCP per-connection, and use a static IP instead.
In P, you still can't switch the DNS to a regular server as far as I can see, but they have added support for "private DNS", which I think is DNS-Over-TLS.
This app will allow users to switch their DNS to Cloudflare even if they're on a network where they can't change the DNS settings. For most people, it doesn't really offer much benefit (relative to the effort required to install the app and enable the VPN every time), but in some countries it may allow censorship to be avoided, and would therefore be a good download.