They are Statewide Mutual Aid frequencies for Police, Fire and Rescue to use. Most towns will utilize SPEN1 to broadcast an alert for area towns to receive. SPEN1 is the most commonly used with municipalities, SPEN2 is the Nationwide Calling frequency, it's sometime used by municipalities to talk to neighboring towns in other states. SPEN3 is commonly used as a common car to car frequency (but I’ve rarely hear traffic on it) and SPEN4 is a common frequency for car to car but also fire to pd, fire to ems, ems to pd, etc. If you're traveling in NJ, and don't want to hassle with programming the areas frequencies, at least have SPEN so you can hear if anything major happens in the area.
Mike