Copy/pasting the same code is redundant and updating copy/pasted code slows development velocity. Mixins are reusable chunks of code that are included, similar to calling a function, instead of copy/pasted.
Mixins have some nice features:
In this lesson we learn how to DRY up the code with the SCSS @mixin directive and make copy/paste a thing of the past.
mixins are a little too complicated to teach at 4000mph