I am by no means a historian but from my limited understanding they were looking for what any country is always looking for, more land and resources. One of Hitler's main talking points was the concept of 'lebensraum' that is, living space. This was a man who saw his nation partly driven to capitulation in the first world war because Germany ran out of food and resources (among a variety of other reasons)

Think of it like America's "Manifest Destiny" there's a bunch of land near you, you don't particularly give a shit about the safety or well-being of the people occupying it and you've got a military force that the native population can't even hope to contain or defend against. Why wouldn't you try to take it?