I never understood the idea of having Perseverance collect rocks. Collecting rocks wasn't the hard part. Anything capable of getting to Mars and then returning

I never understood the idea of having Perseverance collect rocks. Collecting rocks wasn't the hard part. Anything capable of getting to Mars and then returning to Earth would be able to do some rock collecting in the process. I'd always assumed I was wrong about that, and it was somehow a significant benefit to having them pre-collected. But I wonder what else we could have added to Perseverance if they'd omitted the rock collector.