On sustainability, it is academic: one needs to be able to create air, obtain water, and produce food; and one needs to be able to repair the machines that do those things. One also needs to be able to create shelter. There are many designs for these things.

And by “self sustaining”, it will not be completely: advanced technologies such as microchips will still come from Earth.

As for “why”, if you don’t know why, you never will. It is the same question as “Why create art? Why study astronomy? Why do anything that one loves?” Many people look forward to, as Musk, says, being part of an interplanetary civilization. I know that some people don’t understand the point of that, but that is simply a difference of values.