(Idea: product development, the board game.)
I believe teams should start "the work" together. If they don't, it takes more time for other disciplines to understand. And bad requirements feed into bad designs that may be infeasible.
You can't go up the waterfall.
Handoff is problematic.
Let's use an example. There's a team of 1 Product Manager, 1 Designer, and 4 Engineers. They have 3 things they've decided to put in the hands of their users: project A, B, and C.
For both the PM and the designer, each project takes 4 weeks. The 4 engineers take 8 weeks for each project.
The classic waterfall approach is to stagger the work, Which means the engineers start in week 9.
The PM writes the requirements, the designer designs the UI, and the engineers implement the code.
A year later, project A is still not done. Huh?! What happened?
The designer didn't understand why the PM had made some of the decisions. During handoff, the designer also found a gaps in the requirements. But the PM was already working on Project B. It took them a while to get back into the details of Project A. The engineers were invited to handoff, but only one could make it; the rest were still busy working on other projects.
By the time the engineers were ready to start, the designer and the PM had gone back and forth a few times, and the project was not shovel-ready.
If the team had started together, the designer would have understood the PM's decisions, and the engineers would have understood the designer's designs.