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: 
The key is that you are getting the file from several sources (peers) at the same time rather than from one source.

If you have a file that's split into 10 parts and takes 100 seconds to download from 1 server then theoretically you can get the file from 10 peers in 10 seconds (plus some overheads) as each peer can deliver it's part in 10 seconds. In practice it will take longer as each peer won't be perfect, and you'll probably hit your bandwidth limit (but you get the idea).

As you have to reassemble the file anyway you don't have to fetch the pieces in order. So if you only have 5 peers with each delivering 2 parts these can be random parts of the file.
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.