As far as I can tell, the US Intelligence community has never explained it's aims/goals for Tor. The fact that Tor not only attracts the type of traffic that US Intelligence would have a lot of interest in monitoring, but also by design then funnels that traffic through a small number of exit nodes, makes it seem self-explanatory. But I wouldn't want to presume anything.
[0] https://en.wikipedia.org/wiki/Tor_(network)
[1] https://www.documentcloud.org/app?q=%2Bproject%3Athe-tor-fil...