What are CMI files used for? Are they only used for migration from uniface 9 to uniface 10?

From the documentation:

"Compiled module information (CMI) provides summary information about the ProcScript modules that are compiled in a component, and where they are defined (for example in the component itself, or in a modeled entity, or IncludeScript).

In Uniface 10, this information is generated by default and is then visible in the Compiled Module Inspector in the Uniface IDE."

The last line defines the primary use with U10 i.e. it show you exactly which code has been compiled into you component and provide quick navigation to said items.

You can find information on the compiled module information at https://documentation.uniface.com/10/uniface/howUnifaceWorks/compilation/CompiledModuleInfo.htm .