RethinkDNS combines three functions that normally require separate applications: encrypted DNS resolution, a firewall controlling which applications may reach the network, and WireGuard tunnel support.
The reason to combine them is an Android constraint rather than a design preference, because the operating system permits only one application to hold the VPN interface at a time, so a firewall and a tunnel cannot ordinarily coexist. Handling all three internally resolves that conflict, and it is the main practical argument for the application over its individual alternatives. DNS queries are sent over HTTPS so the resolver is authenticated and the queries are not readable on the local network, and filtering draws on a large collection of blocklists covering advertising, tracking, and known malicious domains, applied across every application rather than only inside a browser. The firewall works per application and per network type, so a given application can be permitted on a trusted network and blocked on mobile data, and live connection logging shows what each application is actually contacting, which is frequently the most informative part for anyone who has not looked before. Multiple WireGuard profiles can be configured with split tunnelling, so some applications use a tunnel while others do not.
The breadth is also the cost. It occupies the VPN slot, so nothing else can use it, and the amount of configuration available means a poorly considered blocklist selection will break applications in ways that are difficult to attribute.