This pattern library serves many purposes. Among them:

It defines our design system.

By defining our visual design in a front-end context, we are less likely to have any discrepencies between graphic design and what has been coded.

It's a central repository for front-end code.

This repository (or forked and updated versions) will be able to be pulled into the final app. Since everything is coming from one place, there will not be any reduncies and we will not need to worry about keeping things up to date.

It serves as front-end documentation.

Our front-end guidelines explain how our code is written. The pattern library itself defines what each of the patterns have a code sample where you can copy & their HTML. Ideally, one could use this pattern library as a palette in which they could pick and choose patterns to create a page with.