Since there is apparently some confusion what mason is and what it is used for, a brief explainer.
As opposed to being a plugin manager, mason 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 an 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.
This serves the same function as VSCode's package manager, VSCode just happens to call theirs 'Extension Marketplace'; it puts external binaries in central location and plops them on your editor's runtime path.
Each to their own but the whole package manager thingy confuses me so I just use Helix instead... I like how a lot of things are built in, like LSP support
Helix is great but the rigidness on Selection -> Action editing methodology is a non-starter for me. And it isn't because of qualitatively better or worse reasons, it is about mobility and universality. I need an editing scheme that is universal for maximum mobility.
I understand the creator's viewpoint (Helix was made the way he likes it), but I think it would have been better to separate out the editing logic from all the other great, overlapping features.
It would be nice to have a modern, modal text editor that allows a user to utilize an uncompromising vim mode, emacs mode, kakoune mode, or any other custom mode they'd like -- all built in natively. An editor built for everyone should not be opinionated about how one edits.
That along with things like a built in LSP, plugin management, and extensible scripting support (allowing the use of plugins would multiple ecosystems) would be really sweet.
To be fair to Mason, the answer is mostly listed in the first line of the introduction.
mason.nvim is a Neovim plugin that allows you to easily manage external editor tooling such as LSP servers, DAP servers, linters, and formatters through a single interface.
I still use Packer, so perhaps that is not true for Lazy, but the other package managers handle Neovim plugins, mostly GitHub repositories with vim or lua codebases.
Mason installs and manages things like Language Servers, which are separate binaries running on your machine, and also starts them on demand.
I consider Mason integral to Lazy, some plugins require additional tools that should not be bundled like linters and formatters, and Mason is the best way to manage these.
As opposed to being a plugin manager, mason 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 an 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.