Harry's Firebolt

In Prisoner of Azkaban, Harry's Nimbus is broken by the Whomping Willow, and Sirius replaces it with a Firebolt, which is The Very Best Broom Ever In Existence.

So why does Harry need to have The Very Best Broom Ever In Existence? He's a teenager, and he's been clearly established as an Exceptional Flier. The fact that he's an Exceptional Flier almost negates the need for him to have The Very Best Broom Ever In Existence - he should be able to do just fine on any decent broom.

And what has the Firebolt done for Harry anyway? Getting past the dragon in the Triwizard Tournament, flying from Privet Drive to Grimmauld Place in OOTP, and a bit of Quidditch.

In the dragon scene, the emphasis was very much on Harry's flying skills. It wasn't even mentioned that Harry had The Very Best Broom Ever In Existence - he succeeded because he's an Exceptional Flier. Goblet of Fire seems to be a very random, disjointed book, and I think the reason for this is that it was setting up a number of future plot threads. For example, Harry's training for the maze task is what gave him the expertise to lead the DA, and ultimately to defeat the Death Eaters in the Department of Mysteries. The book also introduced Fleur Delacour, who is all teed up to become a character in her own right, and Viktor Krum, whose presence set up all the relationship drama in HBP. So I don't think the dragon was an end in and of itself. I think it was more intended to emphasize the point the Harry is an Exceptional Flier.

The flight to Grimmauld Place is unexceptional. No emphasis is made on how Harry's flying skills or the quality of his broom came in particularly handy. He was competent and able to keep up with the adults, but there was nothing really difficult to it - it was just getting from Point A to Point B. Worst case, it's meaningless. Best case, it's intended to show that Harry can keep up with adults in standard flying for transportation purposes.

Quidditch is the only situation where the fact that Harry has The Very Best Broom Ever In Existence is emphasized, and it is emphasized along with the fact that Harry is an Exceptional Flyer. However, I don't think Quidditch has much significance in the larger plot. I read it as originally intended to increase Harry's sense of belonging in the wizarding world (since he's exceptionally good at Quidditch but not particularly good at anything in the Muggle world), and in later books it was intended to enable various interpersonal relationship plot points. J.K. Rowling has said that she wrote her last Quidditch scene in HBP, so I don't think it has any significance in the overall Harry vs. Voldemort plot, with the possible exception of establishing that Harry is an Exceptional Flier flying The Very Best Broom Ever In Existence.

So far, the Firebolt hasn't proven particularly necessary. With the possible exception of one or two Quidditch victories, a Nimbus would have done just fine. But Harry's Nimbus was gratuitously destroyed and replaced with a Firebolt for no yet-apparent reason. I think some seriously hardcore flying is going to be involved in Harry's defeat of Voldemort.