I've just read a bit about Apollo 13, and 1 question that kept popping up in my mind when they talked about the docking between the Command module & the Lunar one is that why did they have to be apart in the 1st place?

I guess there must be several reasons for NASA to separate those parts and risk a difficult maneuver in space, not to mention all the engineer design for it to happen; instead of just put them together on the pad.

Maybe it's about the balance of the rocket? The force at launch may break the joint if connected already? The size limit of cargo at each stage of the rocket? Something else? I have no idea.