The rod and reel of a spinning combo work together.
For me a lot of has to with casting accuracy. You can be more accurate with spinning than with a baitcaster. You have more control as to were the lure or weight will land.
I do a lot of light line finesse fishing and for light line sensitivity you cannot go wrong with a spinning rod.
If you watch the pros on TV, you will see more and more are using a spinning reel. Most use it for small lures or small plastic baits. Again you get better presence with the smaller baits using a spinning reel.