Mars has a lower escape velocity compared to Earth. It also has a less dense atmosphere so there is less air drag keeping you from hitting higher speeds. Lower gravity might also make some aspects of heavy industry easier - ships and trucks being able to handle more tonnage, comes to mind.
There probably aren't as many environmental issues with strip mining asteroids and the moons of gas giants, because there are no humans or pandas there (yet). You might also want isotopoes of hydrogen and helium that are rare on Earth from those gas giants for nuclear fusion.
So if you are mining asteroids, the gas giants or their moons, and you want to get those raw materials to Earth's markets, it might be cheaper to take those raw materials to Martian factories for processing and maybe even assembly of heavy finished goods, before launching those goods to their destinations.
As to why you might prefer to have your factories in rocky planets closer to Earth over spacestations near the Asteroid Belt, I can think of two reasons relating to operational costs: you don't need to generate as much heat for your humans and you don't need to generate great centrifigual forces to simulate gravity - though I'm not sure how much gravity you would need just to keep feet on the ground and wheels on the road. One additional capital cost that a space station would have would be having to assemble the ground itself.
Mars might still have to compete with other bases like the Moon (closer to Earth) and Ceres (inside the Belt). One interesting advantage Mars might have is it's orbital eccentricity: the planet's distance from both Earth and the Belt changes quite a bit, so you might hitchike with Mars, within the confort of a Martian hotel room, and save yourself just a bit of the hassle of spaceship travel.