Empires have been built throughout history. Apart from those that are built on the basis of religion, what would be the real reasons for their expansion?

1.Would it more of material greed or more of noble visions of the leaders? For example to achieve unity amongst nations, to provide prosperity for humanity, etc?
2.To enhance the expansion, the conquerers have to enlist the conquered people as part of their army. How would they be sure of the loyalty of the soldiers?
3.In the particular case of the ransack of Baghdad, the Mongols basically destroyed the city and its people. What much benefits would be a destroyed city to the Mongols? Also the actions would bring much hatred and enmity amongst the survivors. Would it not be much better to only attack the vital parts of the city?

I'll be glad to critique your answers.