snaps provide a way for the Certbot and its plugins to be packaged, distributed and installed in a relatively foolproof way for ~all Linux distros. This avoids some issues which have come up historically: broken Python installations on the system, lack of updates/upgrades and automatic upgrade (esp. on "stable" distros), no automatic cronjob, reliance on external packagers.
The pip instructions were later added for when snap isn't an option (like with your DNS plugin). It is more involved to setup, lacks automatic upgrade and setup of the cronjob, but is still pretty good.