This is some advice that I picked up years ago. The idea is that having extra material, and different types of resources, lets a project grow and evolve beyond its initial concept. Photographs of textures can provide backgrounds for prints or text layouts, photos that don't make the cut as art can still provide needed depth or context; audio can become elements for a soundtrack, an interview can provide a body of text or captions; collecting maps and other physical things can add new creative opportunities, and of course there's much, much more that's possible. How much more? I have no idea. That's the point: gather media, with as much variety as possible, to provide for needs that aren't even imagined at the outset.
It's a great idea. I even have "gathermedia" as one of the motivational slides that rotate through as a screen saver on my laptop. As a very linear thinker I tend to become focused on a single idea and have a hard time remembering to back up, go wide, and look for something else in parallel to my expected goal. I'm getting better, and am learning to look for layers, but it's still very much an aspirational goal for me.