VT, to me there are two reasons one might invest in a PID.
Improved temperature control. A PID will reduce the temp swings of a Bradley from 20F +/- your temperature set point to a swing of 2F +/- the set point. A level of control useful for smoking sausage, fish, and cheese.
To provide a temperature control system suitable for modified heating systems than draw more power than a stock Bradley. Some examples would be the dual element modification or single element mods drawing 1,000 watts or more. Several forum members with the Tim Taylor gene have made these types of smoker mods.