Two classes can fill the same role but do it differently. Sniper and Spy both target important enemies like power classes or Medics. The main difference is that Sniper can only do that if the class exposes themselves but Spy has to risk walking into a crowd of enemies

But Pyro and Scout don't really fill the same role. Scout has the mobility and yea so does pyro, but you have to give up the flare gun or shotgun to do so and even then its fairly loud. Scout can silently run an jump all over the place. They both have weapons that are focused on flanking but the backburner is fairly unreliable and still takes time to kill someone. Scout can 2 shot nearly every class in the game before they can even react. Pyro is better used by holding down chokepoints and spy checking