Sorrow,
Everything could be considered redundant and pointless once you have a fighter, bomber, transport and strike aircraft. Why have anything besides the P-51D, B-17G, C-47A and Il-2? That covers all categories.
The reason is that it makes the game more interesting. The Pe-2 is worse than the Tu-2, but some people would rather fly the Pe-2. The Pe-2 is more useful in scenarios as the Pe-2 was used through more of the war and in vastly greater numbers than the Tu-2.
Why add the Mosquito FB.VI? The P-38L is better in nearly all ways. How about the Spitfire MkVb? What use does it have in an arena that also has the Spitfire MkIX? The P-47D-11 and P-47D-25 don't bring anything to the table that the P-47D-30 doesn't.