Making detectives reactive and assigning them to cases reduced the likelihood that:

Making detectives reactive and assigning them to cases reduced the likelihood that "detectives could trick people into committing crimes".

They do the legworks which watch officers don't have sufficient chance to deal with. Bigger divisions with more assets may have diverse detectives for various sorts of violations, for example, a theft detective, murder detective and so forth. Private detectives or examiners are not sworn cops.