How does Blockchain as a Service work?

[Wikadin]

I was reading an article from the recent MSDN magazine about blockchains on Microsoft Azure (Blockchain as a Service, BaaS as they call it). It seems that the platform gives the possibility to create a separate Ethereum blockchain from its own genesis block that evolves independently of the main Ethereum network. Do you know how it works? Why would somebody choose to create new blockchains and not to use the main Ethereum ledger?