The more you upgrade the building, the more features and additions you'll get.
Such features will appear inside the upgrading screen's box when you upgrade a level.
For example, increasing the building's level from 1 to 4 unlocks missions relating to the four factories within your base.
Furthermore, improving this building is considered a necessary condition for upgrading the other buildings inside your base as well as finishing the research linked to the iron blood relic in the research institute.
Such features will appear inside the upgrading screen's box when you upgrade a level.
For example, increasing the building's level from 1 to 4 unlocks missions relating to the four factories within your base.
Furthermore, improving this building is considered a necessary condition for upgrading the other buildings inside your base as well as finishing the research linked to the iron blood relic in the research institute.