IM2 was pretty much a big trailer to the avengers
the whole movie was tony stark proving to nick fury that he was mature enough to be part of the team
a lot of world building, intro to howard stark and the history of the MCU. Two eventual avengers (BW and WM)
further developing his relationship with pepper
it was far from a great movie but if you are to marathon all the MCU movies you have to include it to get the full story. you can leave IM3 out and never notice anything missing, even avengers AOE completely ignores the ending of IM3
I mean that's not really saying anything. IM2 was made to build the world like you said. IM3 was meant to be a more stand a lone film. Has nothing to do with which one is more enjoyable. BW's intro was cool. Nobody cared about WM. And Hammer is only appreciated for that little dance he did before the Expo began. Other than that, movie was just downright bland.