Depends on the nature of the Empire. In fact they where always more predefined and agreed upon than it looks like. The Roman's for example did not conquer all of Germany or Great Britain, simply to show what an Empire brings. So the normal Empire has a predefined frontier, however an Empire is always involving multiple people and countries.
Then there are things like Khanates or Imperium, which where more licence to conquer for a certain area and in this sense there for that. Here the empire adds an additional area into their Empire and prepares it through a different ruler.
Also one need to see war as a form of politics in the last centuries rather than the demonic thing it is now. Especially when a war was fought with swords and on horses many people always liked times of wars and further could gain good rewards when the Emperors let them conquer some land.
Sometimes an Empire ended and the structures of power needed to be removed, and shaken like Napoleon did with the HRR. This is seen as conquest even he just acts in the name of the HRR.
Pls give me a brainliest if this helped thx
