ADS-B in without ADS-B out is almost useless for traffic. It is good for Nexrad radar if you remember the radar images you are seeing could be as much as 20 minutes old. It is also good for receiving metars.
ADS-B traffic is received two ways. With ADS-B in, you can receive any traffic in the vicinity that has ADS-B out and is transmitting on your frequency, either 978 or 1090. Generally, 1090 is for aircraft that fly above 18,000 feet and 978 is for aircraft flying below 18,000 feet.
All radar traffic with mode C or certified ADS-B out is transmitted via the FAA ground stations. However, to receive this ground station signal, you must have ADS-B out to trigger the ground station. If you don't have ADS-B out, you may sometimes be able to intercept a signal being transmitted to another aircraft in close proximity that does have ADS-B out.