It's a stepping stone to building very large spacecraft/space stations that have rotating habitat rings that would be used to colonize the solar system. These

It's a stepping stone to building very large spacecraft/space stations that have rotating habitat rings that would be used to colonize the solar system. These kinds of things can be put into cycler orbits around the Moon or Mars[0] which would allow relatively comfortable and cost effective journeys to these locations. As the scale of these kinds of craft expand they could eventually become destinations themselves akin to massive cruise ships in the sky where some people live out their entire lives. Biosphere 2 was obviously a flawed experiment but it seems that Mir, the ISS, and Tiangong have shown that long term habitation in micro-G is possible, it seems like the extension of that is to try long term habitation of a body with gravity and material to experiment with. [0] https://en.wikipedia.org/wiki/Mars_cycler https://en.wikipedia.org/wiki/Mars_cycler