First, let's talk about how quidditch doesn't make a lot of sense. The snitch is worth 150 points and as soon as it's caught, the game is over, regardless of what else happened. The obvious criticism here is that everyone should just look for the snitch, get it found as quick as possible, score the quick 150 and be done with it. So there's obviously something more going on with this silly game. Let's look at it more closely.
The keeper functions like a goalie in muggle sports, except that they are sort of expected to fail. Most muggle sports with a goalkeeper have very low scores. Soccer and hockey commonly have scores of 1-0, for example. But quidditch often has combined scores in the hundreds, meaning an awful lot of those ten-point quaffles go through the hoops. So the keeper doesn't really stop the other team from scoring, they just stop the other team from scoring too much.
The Chasers and Beaters
These guys are where the action is. Their job is to basically fight with each other and score as often as possible. They have weapons. Despite the fact that there are 700 ways to commit a foul in quidditch, there seems to be very little controlling actual gameplay. There is strategy involved, and they have preplanned plays, but for the most part, quidditch is barely-controlled chaos.
The seeker is the position of glory. Front and center in team photos. Cheered for when they catch the snitch. Players in this position are elevated to celebrity status multiple times in the series. The seeker's job is to just catch the snitch. It doesn't matter what else is happening. They don't help score goals. They don't help guard the goalposts. They don't get involved in anything else. They're small and light - often the opposite of what we think of as an athlete.
Krum and The World Cup
In GoF, Krum catches the snitch at the wrong time. The game ends, but his team loses. Even though the seeker did exactly what he was supposed to do (he tricked the other seeker into injuring himself, he stayed focused, he caught the snitch), he did it at the wrong time and his team lost as a result.
Harry and his Quidditch Record
If Harry plays and finishes a game, his team wins. Rowling does a good job disguising this by giving Harry disappointments in other forms immediately following some of his wins (like losing all the bones in his arm, or getting a lifelong ban from Umbridge). If Harry plays, but doesn't finish the game, his team loses.*
Now the Metaphor
Harry is the seeker in the series. At first glance, he's unqualified for the job. He's young, raised in the muggle world, and more than a little unstable. He's the opposite of what most would picture as a wizarding hero. His job is not to get involved in the main conflict - the barely controlled chaos of the fight against the Death Eaters - but to seek out the horcruxes and destroy them. He has to finish this job, or his team will lose. We also find out that he has to truly finish the job by sacrificing himself; if this is done at the wrong time (say... if there were still five horcruxes out there), his team would lose.
To take it one step further, Ron is the keeper. I don't think it's a coincidence that he actually plays keeper for Gryffindor, very poorly at first, but then grows in both skill and confidence, after years of wishing he could be on the team. Ron does the same at Harry's side. He wants to be there but gets knocked out by a giant anthropomorphic chess piece, gets blocked out of the chamber of secrets with Lockhart, has his leg broken by Sirius-as-a-dog, and is merely an observer of the Triwizard Tournament. Eventually, though, he fights at the ministry, he battles Death Eaters when they infiltrate Hogwarts, and he fights the good fight throughout DH. He's imperfect, he fails a lot, but he does his job well enough that the other team doesn't do enough damage to stop Harry from winning in the end.
Without Harry doing his job, and doing it right and finishing it, the anti-Voldemort movement has no chance. Even Dumbledore couldn't resist the lure of a horcrux when he tried to destroy it, and it nearly destroyed him. Harry, the seeker, is the only one who can win the game. And he can only do it if he has a full team of people and their weapons fighting the other team, keeping them at bay just enough for him to finish his job at the right time.
*Yes, his team sometimes wins without Harry playing at all. I say this is a function of Rowling trying to tell an interesting story. They have to play a certain number of quidditch matches each year for the sport to make "sense" to us readers. Yet she finds a lot of ways to make sure she doesn't have to write the actual scenes on the page, since she admitted to hating the task of doing so. 1. Why include the sport if you hate writing it? Must be some other reason for it to exist... 2. Having Harry sit out multiple matches provided a lot of character conflict, helped with pacing, and relieved the monotony of sitting through what is basically the same thing over and over again as a reader.