SureKill Exterminators was, supposedly, a plague extermination company sent to the Hyperion Hotel to eliminate bugs. Truthfully, they were sent by Gavin Park of Wolfram & Hart to plant the place with electronic bugs. The only place that wasn't bugged was the janitor's closet; Cooper was found and expelled from the Hotel premises before reaching said closet.
It is unknown whether SureKill was a functioning company that actually provided extermination services, or merely a front for Wolfram & Hart to engage in illegal surveillance.
Thanks to the tapping, W&H was able to spy on Angel Investigations for weeks, which allowed them to discover Darla's pregnancy and oversee Commander Burke's operation and subsequent death the hands of Daniel Holtz and his Grappler demons. However, Lorne discovered the transmission, as he could hear the devices' humming, which allowed Angel to set a trap to distract and destroy the W&H's Special Ops Team, while the rest of the Angel Investigations staff took Connor to safety.