Sports are an example of where I need multiplayer. The only way they can make Madden against the computer hard is by making opponents just arbitrarily blow through tackles, making (bad) opposing QBs have magic awareness of the field and silly accuracy, etc.

Playing other humans online allows a level playing field (well, if I didn't use the Patriots this year) and makes the chess match of play calling actually meaningful. I can disguise my looks and mix up play calls and have it actually cause confusion, instead of having the computer complete unaware of context.