[Nasrudith]

Really terraforming Mars in itself is peripheral to if we can colonize it. Creating a vast possibility subterranean "bubble-city" is technically also a possibility that would reduce the needed gases by orders of magnitude. It would probably also call for a lot of design work to be psychologically tolerable to its residents long term. The why bother remains the same however - unlikely to pay off directly in itself but the research from trying will likely have other applications. Emergency backup for humanity is likely the biggest "practical" reason.

Technically we could theoretically recreate a magnetosphere and import gases from other sources but calling it recordbreakingly expensive would be a massive understatement. Even attempting it calls for a degree of megalomania.