It would provide proof of concept and troubleshooting for many technologies which Mars colony proponents seem to think already exist and they can just buy or sl

It would provide proof of concept and troubleshooting for many technologies which Mars colony proponents seem to think already exist and they can just buy or slap together on short notice. If nobody can manage a self-sufficient biodome on Earth, there is basically no chance of it working in a much harsher and demanding environment. Mars is a harsher environment than actual empty space, you can't just toss a space capsule onto the surface and expect to live in it for the next 2 years even if you had the food and power and extra parts.