Why were those installed in the first place?

Some are installed by private entities. Home Depot installs Flock cameras in their parking lots.

I assume their primary use case is combating organized retail theft rings, as companies like Target spend a great deal on this problem (to include famously having their own accredited crime lab).

Speaking only for areas near where I live, it was in response to a persistent uptick in home invasions. Police can't be everywhere at once, and LPR cameras flag stolen cars and mismatched plates that thieves like to use.

Harvest data and let the techno fascist state that is slowly emerging figure out a use case later. For potential scenarios: if you like sci fi you can watch minority report, if you like history you can look at central Europe around 1930