The pertinent bit for people who read this website will likely be this:
The access to such material varies, but much of it passes through an NSA department responsible for customized surveillance operations against high-interest targets. One of the US agents’ tools is the use of backup files established by smartphones. According to one NSA document, these files contain the kind of information that is of particular interest to analysts, such as lists of contacts, call logs and drafts of text messages. To sort out such data, the analysts don’t even require access to the iPhone itself, the document indicates. The department merely needs to infiltrate the target’s computer, with which the smartphone is synchronized, in advance.
Basically, the NSA has figured out that
~/Library/Application Support/MobileSync/Backup exists, and that if they have access to a target computer, they can extract content from this directory. This is yet another reason to encrypt your iPhone backup.