Why is it needed to send a person to Mars one day when we have Rovers? What not obvious advantages do humans have in space exploration over Robotics and vise versa?
The underlying purposes of NASA sending so many missions to Mars are twofold. One reason is that these missions are laying the groundwork for an eventual human mission to Mars. NASA explicitly labels its missions to Mars as human precursor missions. The amounts of money NASA has spent on missions to Mars far exceeds the amount it has spent on missions to all other solar system objects beyond the Earth-Moon system, by far. This is quite intentional.
Even more importantly, NASA's missions to Mars are a form of soft diplomacy (or soft power), with a not so subtle hint toward hard power. It's as if the U.S. is broadcasting to its enemies and its allies, "We can land a probe with pinpoint precision on Mars after traversing several hundred million kilometers, something no other nation has done. Think about what that means our missiles can do to you."
The scientific returns? They're tertiary. The scientific returns from sending vehicles into space that do not look Earthward are paltry compared to the vast sums of money such missions require. Think of how many medical science PhDs could be funded by one mission to Mars. When we (we meaning humanity, not necessarily NASA) do eventually send humans to Mars, that the geologist amongst them will take seconds to identify a rock as volcanic compared to the weeks it takes with robotic probes, that will also be tertiary. What will be firstmost and secondmost in importance is that humanity will have spread beyond the Earth.