Tow package usually gets you stuff that's hard or expensive to add on later. It varies with manufacturer and model but I've seen the following on tow packages:
Bigger brakes
Added or bigger transmission cooler
Bigger radiator
Bigger alternator
Variable load flasher
Heavy duty shocks
Wiring for the trailer, including the controller connector and wiring to the back of the vehicle
Plus the receiver.
A pretty good deal for the $150 or so they usually charge.