What do you mean by 'plugs in'? Is it just that Atom has good support for syntax highlighting or something else?
You can add them for things like syntax highlighting, but they can do so much more, and in many cases can border on "IDE-level" features.
Things like autocomplete, code linting, smart refactor tools, find reference/definition, test runners, debuggers, git integration, and a ton more. (one that I use a lot adds a terminal in the browser so I can run some command line tools without having to tab away to something else)
They are pretty limitless. If it can be rendered via HTML/css/js, it can be a plugin.