Why did Hitler bother occupying every country he conquered? Surely fighting with partisans in the Balkans was something that could wait.

I understand the importance of occupying places like Belgium, France, Italy (later), and some eastern countries for the purpose of protecting your borders and exploiting/utilizing the resources of the countries that were conquered for the war effort, but did they really need to get involved in Yugoslavia, Albania, Bulgaria, Greece, etc?

Wouldn't it have been wiser to skip over some places and let the threat of invasion suffice to get whatever cooperation or resources they needed from a particular place?