Question: Why is it said that BitTorrent is faster, partly, because it transfers random pieces of a file instead of transferring it from start to finish in a contiguous/linear fashion?

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**EDIT:** But why do they *have* to be 'random'? Why not a more predictable 'semi-contiguous' transfer in which one peer gives you the first half, and the second peer the other half? This would theoretically reduce overhead.

Answer: 
Regarding the edit asking why they have to be random: Let's say that torrents were changed to only download the first block that the downloader doesn't already have, thus guaranteeing a contiguous transfer.

I then share a 100MB file. 10 people connect and each downloads 50MB before I go offline.

These people are then screwed, because they all have the first half of the file, while only I have the second half. Until I make it available again, they can't complete the download.

Instead, with the blocks being transferred in a random order as is actually the case, each of these 10 people has a different set of blocks, so they can trade among themselves in order to complete their downloads without needing me to be around.

Even better, though, because each of them has a different set of blocks, each of them can download from *all* of the others to fill in the gaps in their copy. Your edit alluded to the idea of starting from a random point in the file and downloading contiguously from that point, but that would increase the amount of overlap from one downloader's copy to the next (if Alice starts at the beginning of the file and Bob starts 10% into it, then there's 40% of the file that they both have and only 10% unique to each of them that they can trade without me reconnecting to make the rest available). Downloading in a random order maximizes the uniqueness of each downloader's set of blocks held, thus maximizing the ability of downloaders to start trading among themselves and maximizing the odds of it being possible for them to assemble a complete copy if no seeds are available.

A single contiguous download may (or may not, depending on conditions) be better for *you*, but downloading in random order is better for the network as a whole.
Most responses don't seem to be answering your question.

**BitTorrent is not faster.**

It's actually *slower* because of the overhead of connecting to multiple sources.

The real difference in practice however, is you are downloading from many sources that can **potentially add up** to be faster than what a website could give you. Many web servers are under heavy loads or are otherwise speed-limited so you don't suck them dry.

The fact that BitTorrent transfers the file in a seemingly random order has no direct affect on its speed.

In regards to your edit, the **pieces are not random!** They are prioritized based on rarity: a rare piece needs to become less rare as fast as possible so it is not lost if the source that has it suddenly leaves. It has to do with availability, not speed.