TBF most of the places where starship will go have already been explored. I think spaceX's plan is more about exploitation
As a comparison, Opportunity rover covered 45.16 km in 15 years. The Apollo 17 crew covered 35.74 kilometers in 3 days.
It is just that we are not found of sending people and letting them die there, so a human mission implies a return mission, and if there is a return mission, of course you want to bring back souvenirs.
In fact, a sample return mission can be seen as a step towards a manned mission. First you try to bring back a pile of rocks, then you consider bringing back humans.
And yes, humans are far more efficient than robots at space exploration today, but robots keep improving, and I think it will take many years before we put people on Mars, so by the time we are ready for a manned Mars mission, we will probably have much better robots. Not as good as humans, but digging lots of rocks and moving faster than a snail will probably be well within their abilities.
Clue?
Perhaps an answer is that one of the contingencies being covered by the Perseverance sample-collecting is against future contamination? Maybe the NASA missions have been extraordinarily careful about not contaminating Mars with earth microbes, but they're worried that future commercial crewed missions won't be able to be so careful. Then when the humans get there, they'll have some unspoiled sample containers to experiment on and compare to the post-human samples.
Making a successful return trip would be an important milestone, we never did anything close to that before. Yes, moon landings, but the moon is 100-1000 times closer and 10 times less massive than mars, with half its surface gravity.
If it ever happens, the "get back samples" mission is likely to focus on that technical aspect more than anything else, it may not even have a way to explore and collect samples. So let Perseverance do the collection because it is what it is designed for and put them in a neat pile so that the other mission can focus on getting them back.
Further missions should be able to do both collection and return, maybe even take people, but starting small may be a good idea, especially if human lives are at stake.