If human beings can expand to other worlds, or live in space easily, or even just access enough resources to stabilize at 10B human beings with zero poverty - then we can turn the Earth into an Eden - so conceptually nothing could be more environmentally beneficial. At least that’s how the optimistic vision goes.
Arguably, colonizing Mars is easier than uprooting ancient prejudices and dismantling a thick set of special interests that is the politics (worldwide, not just the U.S.)
Space and the surface of Mars are hostile currently and will be early on, but that will gradually improve along with infrastructure. Mars will be easier to improve quality of life on, but space stations or other spacecraft could be made quite nice as well with access to resources like those in the asteroid belt.
If nobody gets through those early rough stages, it can never get better. So from that perspective, the sooner that ball gets rolling the better.
Mars offers no protection and not exactly rich on resources. I think, Venus sky cities[1] have bigger potential. There is a drove of resources plus free energy from Sun.
I think Venus will eventually be very interesting, but the lack of access to its surface means that all mineable materials will need to be imported, which is a big problem.
For getting started on long term self sufficiency in space (which should be a goal, if we’re taking crewed spacefaring seriously) we can’t really do much better than Mars. Once there’s more infrastructure in place for in-space manufacturing other destinations become a lot more practical.