CNC machined parts are usually required in the prototyping process when a casting process is going to be used for the production run.
CNC machined parts have a much shorter lead time, ranging from days to a couple weeks. In the prototyping phase, CNC machining can be more cost effective than a casting process, due to no pattern or tooling costs.
Low volume production: For higher quantities (10 - 10000) of simple geometry or complex parts CNC machining is more cost effective than most other processes. You do not have to pay high upfront costs. 3D files are not required, but do speed up the process.