You don't have to use them for that, you can use them for composing models out of shared bits of functionality, for example 5 of your models have a status - put

You don't have to use them for that, you can use them for composing models out of shared bits of functionality, for example 5 of your models have a status - put all the code about statuses in one place in a concern and include Status in your models, and possibly similar for controllers - this cuts code duplication. There is also an argument for making models smaller by splitting some of their functionality into modules too (what you're talking about), but I think that's a weaker case - better to use concerns for shared code.