We can grow plants on Earth, why would we do it in orbit.
As for some reasons to grow, or manufacture really, things in orbit, landmass is one reason. There is a whole lot of real estate up there that we don't really have access to on earth. There is also the ability to send down materials to anywhere on earth without requiring a huge transportation network that causes things like roadway congestion. On top of those, building facilities that do heavy amounts of nasty polluting become less of a concern. It's not really as much of an issue to dump a bunch of toxic stuff into space compared to say, a river. I suspect nuclear power plants also start to become more viable as there is less of a worry over meltdowns as you're already surrounded by massive amounts of radiation and have no population to evacuate.