To have more slaves and to protect them better. Its like a garage. The bigger and stronger it is the more cars you can put in it and the better protected they are.

Investing in civilian infrastructure increases the tax income produced by the world.  It's an economic investment, plain and simple.  For emergency infrastructure, this is literally building defenses on the planets surface to maintain control even when under planetary bombardment.  Again, investing in protecting assets you control.

Think of it from the perspective of a player; you don't really care about the imaginary people on the planets, you care about the tax income that they produce.  If it's a good economic investment, you build the infrastructure.  If it isn't, you don't.  I don't see any abstraction here at all; it's a means to an end.