Building in Space--> is it really necessary for a CV?

I'm preparing to build my first CV...I think I may end up choosing the default CV tier 2 blueprint and was wondering if it's really necessary to build it in space or not...and why? I mean is it because most of them can't fly in atmospheric planets?

Also, what is the main reason for building a space station? Is it mostly just for having a base in space instead of on land...I mean what are the benefits of that exactly?

Thanks guys