Law enforcement agencies
As I wrote before, law enforcement agencies have a strong presence on the Dark Web. The officers working for these agencies can engage in undercover sting operations to capture criminals and other people engaged in illegal activity. Using Tor provides anonymity and hides the source IPs of the officers' computers, which otherwise would have blown their cover.
In the same manner, law enforcement agencies perform discreet online surveillance of services and websites without revealing themselves, by using Tor.
In order to better explain how law enforcement can operate on the Dark Web, I'd like to tell you about Operation Bayonet, a Dutch-led sting operation, to take down Hansa, a Dark Web market site, numbering 3,600 dealers offering more than 24,000 drug product listings, as well as fraud tools and counterfeit documents. They were able to monitor Hansa's buyers and sellers, using various tools and methods, and discreetly altered Hansa's code to grab additional information...