Understanding the flow and pieces that make up the application can be tough with out visuals to help guide developers, stakeholders and product owners. Starting with simple boxes, outlines and white-boarding sessions, the design gradually takes shape as decisions are made and designs are researched and tested.
Once a high fidelity mockup is completed, I like to take it a step further and build it in HTML & CSS, and sometimes right into an angular project just to see how it presents itself. You can find roadblocks, possible development issues and better user feedback response.
The ability to show the developers exactly what you are expecting can be crucial when time is of the essence. This extra step helps me to better follow and implement the required styling that has been approved by the client.