This set-up is important because it emphasizes the difference between Drake and, say, Lara Croft, especially since parts of the game are heavily influenced by Tomb Raider. Whereas Lara is a trained survivor and a perfect athlete with her own gym in the Croft Manor, Drake is an ordinary bloke who can take care of himself but who'd rather go down tothe boozer for a couple ofbeers than work up a sweat on a running machine. He's just been thrown into a hostile environment that's going to push him to the limit of his endurance.
And, rather cleverly, you can see how uncomfortable Drake feels throughout the game, giving him a believable, vulnerable edge sorely missing from most action game heroes. Think Bruce Willis in the original Die Hard. He gets short of breath after prolonged running and jumping; he mutters "crap!" a lot when shots whizz past his face; he hunches his shoulders and covers his head when he's under heavy fire from the mercenaries roaming the island; and even his hand-to-hand combat skills have a raw, untidy quality since he isn't a trained fighter. From the moment he touches down on the island, you can feel his stress and desperation from being constantly outnumbered and outgunned.