The primary intended purpose of the tor network was to provide cover for US agents. Many of the core developers of TOR have at various times either directly worked for US intelligence or have been funded by them. It was opened to the public and popularized, because an anomization network that is only used by spies is pointless. Most of the current exit nodes are currently located in the US, so there is no question that almost all TOR traffic is monitored by the NSA. The tradeoff is that while TOR makes it slightly harder to identify targets, the majority of them use TOR and there are still ways to identify them if they are not extremely careful (this has been revealed in some of Snowdens documents).