Humans are still generally better than just having a robot there because they're both more flexible and capable of problem solving. Maybe for the same money you could get enough robots there to make up the gap but sending humans to Mars is sexier and attracts more money than you could get for the robot missions. Then even if we never actually go 'preparing' to go to Mars attracts ongoing money for studies on the ISS and for sending robotics missions.

For NASA in particular, they'd be interested in doing science locally, establishing multiplanetary infrastructure, national pride reasons and help people think that the human race is making some kind of forward progress, and to motivate young people to be interested in science. Basically reasons that are similar to the reasons why NASA does anything.

For people in general, I'd say people might want to go to Mars for mining (maybe direct mining of Mars or more likely to serve as a refueling spot with a shallow gravity well for mining operations in the belt), real estate speculation, science, low-gravity sports and entertainment, retirement communities for people with mobility issues, to establish new communities according to their rules rather than working within the established systems of Earth, tourism, curiosity, a sense of adventure, and so on.