[scardine]

Because a magnetic shield will prevent the solar winds from wiping your atmosphere.

Without a spinning core, Earth would be like Mars.

Of course you can live in caves instead of trying to terraform Mars, but doesn't sound so fun.

[AgentME]

Solar wind strips that atmosphere very slowly over geological timescales. If we terraform Mars, then we probably have the technology to replenish it faster than that loss.

[pasbesoin]

Geomagnetism also keeps your genetic change (via irradiation) from exceeding your tolerance. Relevant within single and generational lifespans.

I.e. Breathing is of limited utility, if the solar weather still gives you cancer.

[sosborn]

If we had the ability to terraform Mars wouldn't we just terraform Earth?

[blhack]

...because we don't need to? Earth is already full of plant life. Mars isn't.

Sorry for the overly blunt answer, because I know this isn't the question you're asking, but IMHO the question you're asking is based on a ridiculous premise.

We are polluting the Earth. Yeah, this is really bad for the Earth, and for us, but it isn't going to kill all life here. The thing that a "backup planet" is here to protect against is something like being struck by a meteor, or a plague, or nuclear war. Something where everybody dies very quickly, not something where life changes radically.

[nickff]

If you want to terraform Mars, it's probably not because you decided Earth was beyond salvation; I haven't heard many people wanting to abandon Earth. Most people in favor of terraforming Mars want to have a 'backup' for intelligent life, or want to begin humanity's expansion past the bounds of Earth's atmosphere. The 'backup' motive is comparable to suggesting someone backup their electronic data instead of just trying to get very reliable drives.

I am also not sure that it is possible to 'terraform' Earth. 'Terraform' means 'to make like Earth', and Earth is as Earth-like as is possible, no matter how hot or cold it is.

[thaumaturgy]

Humans have been terraforming Earth for at least the last hundred years (industrial scale) to 10,000+ years (agriculture, livestock, extinction of other species).