The ADL is a private intelligence service. It maintains files on people. It has members who can relay information back, monitor people, etc. It is pretty-much immune from criticism by the mainstream press. It can act to smear people and ruin careers.
As such, it is a valuable resource to whoever controls it. The big question is: who controls the ADL, and what is their agenda?
The Simon Wiesenthal Center is almost as bad. More gun control seems to be their goal. The list of "hate sites" on a CD-ROM the center provided to police, civic groups and schools contained such organizations as Women Against Gun Control.