It may seem a little silly to call fat marker sketches a technique or a tool. The reason for calling them out is we too easily skip ahead to the wrong level of fidelity. Giving this rough early stage a name and using a specific tool for it helps us to segment our own creative process and make sure we aren’t jumping ahead to detail a specific idea when we haven’t surveyed the field enough.
We don’t want to make our early UI flow look too finished. We need to leave room for UI design.
This idea was first introduced by Ryan Singer in 2009 under the name UI flows.
I have followed Ryan Singer to UI flows approach for 10 years and it gave extraordinarily result. I can decide what actions are available on each screen and how each action links to the next screen without deciding how the UI elements are placed.
In 2018, Ryan renamed the UI flows into breadboards.
Wireframes require too many visual decisions too early.
Now, in the book Shape Up, this technique has an "official" documentation.