2001 Saturn L300 Rear bearing question

Good evening, All:
I need to replace the rear bearings on my 2001 L300 but the Timken bearing assemblies I bought don't come with the four nuts that hold the assembly to the car. The Haynes repair manual recommends replacing the old nuts (4 per wheel) with new ones. Firstly, are new nuts an absolute requirement (if so, why)? Secondly, where can I find a supplier for the new ones (Saturn wants $16/EACH). Thanks in advance/PCL