Code review is an integral part of multiple developers working on the same project, whether you are on a team of developers, or contributing changes to public open source projects. In either case, the reviewer needs to be comfortable with your changes before they will merge them.
This leads to one law of code review: The clearer it is to the reviewer what changes you made, the faster the reviewer can review and merge your changes.
Unfortunately, this gets complicated when you’re developing iOS UI. Whether your team has chosen Storyboards, programmatic UIs, or a combination of the two, reading either Storyboard XML or code diffs is really hard to visualize for anything beyond trivial changes. Worse, if you have a combination of storyboards and code — which is often the case — you can’t necessarily trust that the storyboards before and after even look like what the app does.
To make the intent of your changes clear, you need another way to show that diff. For larger UI changes, representing those diffs as either screenshots or video is key in making those changes as clear as possible.