What the units would be I don't know. And how you would be able to use them? Well, I don't know either. It's just that when making a scenario you could need a ... evil enemy hero which the player has to defeat. Then you would be able to use a hero from that pack no matter what techtree and factions you are using. There also could be extra units like ... Glestimals, maybe? you can give to a player or something else. If you would have to add all the units to every faction it would require a lot of memory.
Some scenarios or campaigns could require that you download certain heros or units. Or could just have them in their package.