A reason to go to Mars

--- In spacesettlers, "Marcel" wrote:

> True. But its much easier and cheaper to set up a colony
> on Mars than to build one from scratch.

How and why? In both cases, you have to keep the air in, keep
the radiation out, and regulate the temperature. Why is any
of that easier on Mars than in High Earth Orbit?

Also, wouldn't any colony on Mars have to be built "from scratch?"
It's not going to build itself and sit there, waiting for us.