Why do dictators need to find scapegoats for their nation's ills?

Answer 1

Dictators can gain popular support by blaming the problems of their nation on a scapegoat. In general people like the idea of blaming their problems on someone else. That exonerates them from facing their own personal responsibility. If sacrifices need to be made, people usually prefer for someone else to have to make them.

Answer 2

It helps to unite the people against a common enemy, and add to their power.