Board Thread:Warhammer 40k General Discussion/@comment-26540047-20161113174417/@comment-15803608-20170611044446

If there were no External Threats (Necrons Extinct, Tyranids Extinct, Orks Extinct, Dark Eldar Extinct, etc) then there would still be uses for Space Marines and Primarchs. The instincts that served them well in combat, and the experience they developed over their tenures, make them great leaders of men. I can see Retired Space Marines and Primarchs holding administrative government jobs, managing and leading in times of peace as well as in war.

The alternative is that they could be kept in reserve, training so as to be prepared in the event of a new extragalactic threat rising. Or if the Imperium expands beyond the Milky Way Galaxy, then the Space Marines could act as an invading force to wipe out any filty xenos out there.