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.