For example, if I have a team with designers, they likely know their way around Sketch and Figma and onboarding them onto something like this wouldn't be too difficult. At this point they can start contributing real, demonstrative code that compiles out of the box. Sure it lacks actual functionality and maybe it's a little rough around the edges, but that's not the point.
The point is that I, as one of the few developers on the team (or maybe even the only one) have a greatly reduced workload because now everyone can contribute to building the UI. With frontend out of the way, I can work on backend functionality exclusively and marry the two later on. From the year that I've worked with Flutter, the framework makes it easy to separate those two which further adds to the efficiency gains. Rough edges can be cleaned up relatively easily and the time sunk into cleanup probably adds up to less time than starting from scratch.
As a real world example, I have been building a mobile app for my startup since this summer. My girlfriend, who wanted to get into UI/UX design, is learning the ropes with the material widgets and making demo UIs by herself. With a few more weeks of practice, she will be equipped to start building some of the views in my app without further guidance. This takes a huge load off of my shoulders, and it's really where Flutter shines.