Such a great question!

In an ideal world, we wouldn't have introduced both.

Our design system serves a number of applications. One of the major ones favors 14px body copy over 16px.

We knew that we wanted to support a size smaller than the body text for meta data and components like pills/badges. We also had a hard minimum text size of 12px for legibility.

We introduced 13px to address this and (to help provide clarity) limited the application of this size to certain use cases.

Hope that helps and thank you for reaching out!