Dressrosa opened up a lot of plotlines for the story to progress, the whole kaido saga, thw celestial dragons, sabo getting ace s fruit, the freedom of dressrosa meant the abolishment of shichibukai, disrupting the underground trades, creation of the fleet
While WCI, we got the big meme, the sanji backstory and sh getting a hand on the poneglyph, (the only valuable reason i could think)