Each season has its own primary story, yes.
The following seasons build on the same characters, introduce new characters, and some storylines are extended, while new season-long arcs are focused on. But yes, you could essentially take season 1 as if it were its own show.
Once you get into season 3 though, the higher arc of the series really shows its stuff.
You also view it through different lenses at times. You see the view of all levels of the police department, you see all levels of criminal life and their perspective, you see political, educational, and in season 5, the media.