Cleaners will get rid of evidence like bodies, blood, weapons, fingerprints, and even witnesses in order to hide the proof or notion of the crime -- or, they will 'dress up' the crimescene to mislead authorities. For example, they could make it appear like a break-in gone bad, an act of self defense, a fire, etc.
Elite cleaners are rarely found in the mobs/mafias/gangs etc., but instead apart of a government agency. Considering the drastic consequenced often involved with these agencies actions, elite cleaners are required. You never hear of them, and you rarely hear of what they have cleaned up. All that's left is the event: something happened, but the cleaner will make sure there are no trails left behind. Such cleaning jobs can be on a global scale, such as clearing a trail of money used to get the job done.