[Theory] Completing the Pokedex

I feel this section's been lacking theory threads for a while.

In all Pokemon media, we know that the Professors of every region want beginning Pokemon trainers to help them complete the Pokedex, usually for research reasons. But what other reasons do you think make the Professors want young trainers to help them complete the Pokedex?

Is it so the Professors can get to know the Pokemon better? Or is it for something we don't know about?

Well, I'm pretty sure the proffesors have probably encountered trainers asking for pokemon before the whole starter system, so that's probably a reason why they encourage trainers to go on a journey. Besides, half the population does it. Also, they might have a 'mysterious way' to encounter rare pokemon. They 'might' just meet some legendaries.