some good replies here
remember the fisa warrants ?
they are interested in one person or anyone who has contact with that person and anyone who has contact with THAT person
you keep track of even the" dull" people so the chain is not broken
and they never know when a "dull" person might go rogue–maybe from reading 8chan or something
then if they know everything you ever did or thought a crime can be found or manufactured
personally i think very few major crimes are solved by the tv detective model
if something happens that gets their interest the super fast computer cross references lead them to a result
the only job for the " detective" is making the story apear to have been solved by their superior thinking
remember jussie smollets 2 "dull" friends that no one ever heard of? within 24? hours of being interested in jussie there was vidio of them buying the stuff in a hardware store