Back when I was working at Setoo (a disruptive parametric insurance startup) we've outsourced a design project which mocked an imagined OTA (Online Travel Agency). This was done in order to later be used as a means of demonstrating Setoos capabilities to prospective clients (other OTA's naturally).
The designs were received as a Sketch file and contained perfectly fragmented screen designs - with no interconnection between them.
In Setoo we've been one of the first design teams to recognize the benefits and move to use Figma as our main design tool.
Now that we needed to elaborate, and show various flows and Setoo products on those designs we first needed to create a design system to cater to future design efficiency and consistency.