Some envisioned features or usability gains need to be seen and heard, not just Sketched or Keynoted. A prototype is worth a thousand PNGs.
For example, to gather opinions on different outgoing message confirmation strategies for Siri, I modified Java code on a development server cluster. That was paired with my own Objective-C customizations on the client side.
A different client-side customization I made allowed the team to evaluate several sonic feedback variations before we arrived at what shipped in the 2019 version of the AirPods.