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?

---

**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.
It's an availability issue. If you have to download contiguously, then you're dependent on people hosting more of the file than you. Every person with less of the file than you, will be unable to send you any data. 

If chunks are randomly distributed, the number of hosts you can download from will be higher, therefore you'll download the file faster.