Have a visual before you commit to an expensive build,  helps in a variety of ways, including setting a benchmark or showcasing optimism.

Using established design frameworks for native functional app ,helps achieve this optimism when all user flows are be considered to validate the idea & purpose to stakeholder before any development.

The Output  is and prototype  provided in Overflow | Invision | Zeplin which gives the ability to inspect and copy over the styling code for development in collaboration a specified build framework.