Why is a detective involved in making a deal with states attorney to get out of jail?

It depends on the type of case.  Sometimes they don't want to let you out without checking into your story.  Sometimes they want to see if you were "cooperative" or whether you were a "jerk" to the detective.  Sometimes they don't want the bad publicity involved with releasing somebody before the detective has finished the investigation.  Detectives and State's Attorney's work together on a daily basis, so often it is just a courtesy call.  State's Attorney's want to keep detectives happy, and best way to do that is to not toss away their hard work for an easy plea deal.