Why? The place is an absolute shite hole. Even after a full scale nuclear war and winter earth would still be way more habitable.
The reason to go is not for some kind of back up or alternative to earth. Its a hell hole and near impossible to live there.
We need to go because its a hell hole and near impossible to live there.
As humans tackle harder problems, we need to develop new solutions to those problems.
Going to Mars is all about developing new solutions. The harder the problems, the more creative the solutions will need to be.
Every problem there, requires new solutions that people can solve there, that could help us solve problems elsewhere.
NASA has developed water recycling technologies that no one would have developed if not for them. This could be instrumental over time in low water areas on earth.
At some stage, humanity will need to learn how to access resources off earth, if we want to maintain a sustainable planet. There is an infinite amount of resources off of earth.
Going to space is about going to other planets eventually. Mars is a logical next step