A brief explainer I originally wrote in a hackernews comment.
As opposed to being a plugin manager, mason.nvim is a package manager. It installs editor-agnostic tools like LSPs, linters, or formatters.
As opposed to system-level package managers like homebrew, mason is specifically integrated in nvim. Other than having a UI inside nvim, this entails features such as auto-installing packages when bootstrapping nvim on a new machine.
As opposed to language-specific package managers such as npm or pip, mason includes packages across languages. That means instead of having to install one tool with npm, one with cargo, one with pip etc., you can install all your nvim-related packages just with mason.
While a plugin manager is basically a necessity, mason is more of a nice-to-have tool for nvim. It is convenient for some people, while others prefer using their existing package managers.