Most are money making scams. They earn money by installing spy and other scum ware.
Even the scam ones use the same principle. Instead of one download, they setup 4 or 5 downloads each getting small chunks of the file.
The theory is that mutliple connections have a faster overall throughput than a single connection.
There are other programs out there that use special compression. They work well for large eb-pages, but most msuic, pictures and programs are already compressed, and not compression will make them any smaller.