This is the part where the game artificially pads itself out by adding missions to the main story that do nothing to advance the plot.
There really is no other reason to have Oliver's rescue by a sniper section aside from the designers making it so. Honestly, what I suspect happened is that the designers had already created this segment, and the writers had to find some way to tie it into the rest of the plot. Sometimes, that style produces gems like Uncharted 2. More often than not, it creates segments like this.
It doesn't help that the Sniper Rifle you get in this section is one of the worst weapons in the game. Even the basic sniper rifle is more accurate that the one you receive in this mission. I'm not sure why they did that either.
As for the topic of Jason Dilweed's friends, it's so sad that they have a lot of personality in their backstory, but that never comes up in the main plot. It's such a wasted opportunity. The writers could have used those details to give us some attachment to them. Then, when Jason begins his descent into madness, they could contrast with Jason's new personality, to further highlight how far down Jason stoops. That would've been so interesting, but they didn't do that. That's the problem with this game. The writers missed so many great opportunities.