Long term, the reason to expand into space is to expand into space. The point of going to Mars is not to make money. The point is to find a way for people to live there.

Let go of your sci fi visions of the future. Space colonization does not look like humans getting on a rocket ship on Earth's surface, blasting off between stars, and landing on a second Eden around another star, a world where you can take your helmet off, breathe the local air, eat the food, and drink the water. What we've learned from our exoplanet studies is that the cosmos has exercised infinite creativity in generating new Hells, worlds uninhabitable and lethal to us in ways we never even dreamed of.

Real space colonization is going to take a whole lot of engineering and building. Mars is actually quite habitable compared to the vast majority of worlds in the universe. If you have some basic self-replicating machines, making Mars habitable is quite doable. Cover as much of the surface as you want in vast greenhouses. Live under domes potentially spanning the entire world. Long term, Mars has great potential for human settlement.

As for why? Well I would say it's a moral responsibility. We know if we stay on just one planet, just one star, then it's only a matter of time before some natural or artificial disaster wipes us out, and possibly the rest of life along with it. Earth has the type of biosphere you probably have to scour multiples galaxies to find one of comparable complexity. That's a jewel worth protecting.

We tend to be too hard on ourselves as as species, viewing ourselves as some sort of cancer on the world. At the same time, we are the only hope for life on this planet to survive past a certain point. We are the children of Gaia. It's our duty and moral obligation to spread life as far and wide as possible. Life itself has intrinsic value. And it's our job to spread it.