[Theory] Why I think Oak gave you the dex.

[Oreanna]

Not only that he gave you the dex, but your rival too.
Why? To have the dex crowd-sourced into a more complete one.
Think of it this way:
If more than one trainer gathers data on one Pokemon, the better the info will be for the dex.

 

[Sun]

I'm pretty sure that's already been well-established. The more Pokedexes he gives out, the more information he gets. It's common sense.

This.

The old timer has even made clear he wanted the dex to be filled since he's too old to work on it. Otherwise, we are not needed. Really, there's nothing more or less about it -- the dex holders are there to help gather data for his researches.