The data model is the building. Everything else is finishing.
Almost every custom software project that goes badly went wrong in the first fortnight, when somebody decided how the information would be structured — and got it subtly wrong. Nobody notices for months. Then a reasonable request arrives, and the honest answer is "that would mean rebuilding a lot of this."
So we settle the structure before anyone draws a screen. It's the least visible week of the project and the one that determines what the system can still do three years from now.