Sebastian, inert loaded rounds can be used for a variety of applications. The ones you mentioned of course, but also to simulate proper shape and weight to verify and calibrate ammunition handling systems (autoloaders, conveyors, loading machines, etc.). To have the correct center of gravity, yet be inert, to provide inert examples for new troops to practice handling, loading, etc.
Some of the time, components that are deficient, out of spec, etc. are used to assemble these training rounds, to minimize cost and make use of components that would normally be scrapped.
When I worked for Norris Industries around 1989, my partner and mentor told me that during Vietnam, the Government even bought all the scrap that failed inspection for some reason. We made 2.75 and 5 inch rocket motors, Sidewinder Motors, artillery projectiles and cases, Mortar rounds, and Bombs (Mk. 80 series). Basically rotational munition components.
I have quite a number of Black projos and rounds black with white lettering.
Over the past 20 years I've seen huge numbers of inert loaded 57mm Gun APCAPT and 106mm RR HEP-T rounds.