It means more people get to see the Pokemon identified. Although it seems dumb, they make Ash do this so more viewers can see the PokeDex data for that Pokemon.
For example, if Ash saw a Pokemon 50 episodes ago, and the same one reappeared later, the audience would have changed. The writers make Ash act as if he'd never seen it before so more viewers can experience it. The writers generally care more about what's going to appeal to the audience then anything.
A similar thing applies if he saw the Pokemon a season or two ago, for example if he saw a Pokemon in D/P and saw it again in Best Wishes, the audience would have changed and so would the PokeDex description.