A planet is just a base of minerals and gravity, assuming you can biodome it all. A lot of pressurized volume and plantable surface area is lost though if you have to biodome it. Maybe there's inflatable transparent sheets or plastics or rubbers that can be used on a large scale that could keep pressurized.

If not for living, we might need Mars and other planets, as temporary stepping stones on learning to travel farther, or as intermediate resupply points. Ie if we never go there and never do anything there, we will never progress.