[[Dogfooding]] a design system helps identify gaps in usability and missing components. See how far you can get without leaving the DS, and see what are other parts you are relying on. Sometimes you may be missing a styling API, or a smaller base-level component like a Divider, or Card.
This is a great reason to make a documentation site using exclusively design system components. Recreating the UI as an experiment in tools like playroom is super useful as a quick REPL.
As the DS matures, the gaps should reduce.