Now that you mention it, I think you probably could do nearly everything with just those two. I'm just not used to using a stainless in the oven whereas I instinctively turn to cast for that.

Cast iron has thermal mass. It will cool down the least when food is added. It also will not buckle like, say, a thinner carbon steel pan.

This is important if you want to get the pan hot and for it to stay hot. Common for searing large cuts of meat.

I have all three, for steaks/burgers I exclusively use the cast iron. Just doesn't brown as nicely on the stainless steel. Not sure if the same applies on gas/induction ovens though.