I'm not dismissing it because it's repetitive; I'm criticizing the people that make that argument because they've been responded to over and over again, and yet they still make exactly the same argument while, themselves, ignoring the counterpoints.
But okay, here are a few reasons:
1. Because it's difficult. As Kennedy famously said, we choose to go not because it is easy, but because it is hard [1]. We will need to develop and improve a lot of technology, engineering, and science to get to Mars and build a self-sustaining colony there.
2. The technological developments required to get there and stay there will help further improve life on Earth. A number of technologies have been spun off from past NASA efforts [2] [3]. Mars is a harsh and challenging environment; past efforts like Skylab have been stepping stone experiments towards longer-duration stays in space, and Mars is the next step in that progression. We will learn a great deal about the biological effects of interplanetary travel (necessary if we wish to eventually extract resources from other bodies in the solar system) and agriculture in extremely harsh environments (necessary if we wish to stabilize regions on Earth impacted by global warming). You can see this in action right now: SpaceX is driven to get to Mars, and to get there, they're developing technology that's making it safer and less expensive to get payloads into orbit. If they pull off their planned global communications network as another step towards Mars, that will be a huge benefit for humanity on Earth.
3. It's the only way to begin answering the question, "how rare is life in our solar system?" We already know that Mars had the necessary conditions for life. We don't know if life ever developed there. We don't know life might still be present there. We don't know what forms it may take, how it may be different -- or not at all different -- from life on Earth. One of the oldest questions humanity has is, "are we alone in the universe?", and if we hope to ever answer that, we'll need to venture into space.
4. It will inspire a new generation of scientists and engineers. I think a lot of people assume that scientists and engineers are just sort of a thing that happens. No, they come from dedicated college students, and dedicated college students come from kids with dreams. Science fairs and after-school programs aren't enough; you have to give young kids something really big and really cool to dream about, and then they'll want to be part of that dream. The Apollo program gave us a three-fold increase in PhD students in US universities. You like Amazon? Apollo helped helped provide the fertile ground from which Jeff Bezos found the inspiration to build Amazon [4]. I don't know if the effects of all these new scientists and engineers can be overstated; people say, "we should cure cancer instead of going to space", and I say, "if you want to cure cancer, go to space." I grew up on Neil Armstrong and the moonrise photo and Scotty from Star Trek; I'll probably never get to fulfill my dream, but I got in to software development at the age of 8 because it was the closest I could get to being in a futuristic space ship.
5. Because it is historic. You want to make America great? Do something great. Dr. Zubrin described this really well; in 500 years, nobody will even remember who the President of the US was. But they'll remember -- and celebrate -- the year we went to Mars. Somebody will do it, eventually. China has the resources. India has the potential. Russia has the capability. If America doesn't get there first, I think it'll be time to admit that our great democratic-capitalist experiment has failed. It'll mean we, as a population, have become uninspired and lazy and complacent and lost the spirit of exploration that got us in here in the first place.
6. Because it is possible. We have the technology and science right now to get us there. We just have to try. We aren't still crossing vast bodies of water in wood-hulled ships. Mars is within reach now, so why aren't we reaching for it? What is holding us back? Cynicism? As Dr. Zubrin says, if we can do a great thing, then we should.
7. It will be a "remote backup" for humanity. There are still any number of things beyond our control that could wipe out most life on Earth, set human progress back a thousand years or more. If we expand into space, if we build a self-sustaining colony on Mars before that happens, not only will we develop more technology that will further reduce the chances of that happening, but if it does happen, we'll have much better chances of rebooting civilization on Earth. But it's going to take time for us to develop that far, so we should start as soon as possible.
8. A Mars colony will become a new experiment in human society. Look, there's just no way that a Mars colony will be governed entirely, long-term, by any government on Earth. It's going to need to develop its own governance. What will that look like? Technology is a fundamental part of life on Earth now; how might the next colonial government, full of scientists and engineers, incorporate it into government? What new solutions might they find to the political problems of Earth? We don't know yet. There's only one way to find out.
All of these points have been stated before, more eloquently, by other people before me.
[1]: https://en.wikipedia.org/wiki/We_choose_to_go_to_the_Moon
[2]: https://spinoff.nasa.gov/Spinoff2008/tech_benefits.html
[3]: https://en.wikipedia.org/wiki/NASA_spinoff_technologies
[4]: https://www.theguardian.com/science/2012/dec/16/apollo-legac...