To make sure the data is spread across machines and self validating. Also prevents folks opening chunks to reveal any info. There are debates saying I can tell which chunks belong to a file if I store up all datamaps etc. and that is true, but many ways to obfuscate so even those storing do not know the chunk names. Anyhow that’s another story 
Also helps hugely with up/download of data. Again more info needed but the second last hop can encrypt to last hop so even connected (relay) nodes cannot tell what’s being transferred. It’s an easy addin to the code.
Yes shared with whoever requests it, from say a web link etc.