i suppose its the challenge that GWX brings as the war progresses. My favorite part of the game is not sinking alot of ships, but surviving as long as I can. I have yet to make it into 1943 and have been playing for several years.
The Black Sea and Arctic campaigns are alot of fun.

But yeah, there isnt much variation in patrols. Your job is to sink ships. I sometimes throw in other missions myself. Like getting a radio message on my way back from a patrol that I need to pickup a spy from near some British port.