It's mainly to give us the illusion that we're filling their shoes, as in for example, *we* are Link, or Mario or whatever. You know, pretty much like most RPGs out there.
Personally, I'm not a fan of such an approach, given that later on they tend to give them proper personalities and backstories as time goes on anyways (like official documents and such). --- "Many of you feel bad for this lamp...That is because you're crazy. It has no feelings! And the new one is much better." (IKEA's Lamp commercial)
Because of the Pokestar Studios, the Player Character of BW2 is the most talkative player character in Pokemon. --- 3DS Friend Code: 2449-5500-6430 Name: JMN 3DS Games: Mario Kart 7, Kid Icarus: Uprising