Hitler and the German Luftwaffe targeted British population centers for a number of reasons. The two most important reasons had to do with terrorizing the public and reducing war production.
One reason for the Blitz was to try to destroy or damage factories and other installations that were vital to the war effort. By reducing the amount of war materiel that could be produced, the Germans hoped to be able to reduce Britain's ability to fight.
A second reason was to terrorize the public. What Germany was trying to do here was to make the public lose their morale and their will to resist Germany. This would be another good way to win the war more quickly.
Hitler and his military picked their targets with a view to winning the war more quickly.