Not all airports have towers. If the frequencies you are talking about are at smaller airports and the frequencies are often 122.8, 122.9, 123.0 and a few others then these are called common traffic advisory frequencies and they are used at small airports that don't have a tower. At these airports the aircraft announce their intentions to any other aircraft that is listening so the others can be on the lookout for them and avoid a collision. You will often find situations where several small airports within an area use the same advisory frequencies. I have noticed that in FSX if your are within 60 nautical miles of any other aircraft or airport on the same frequency you are on then you will hear them on the radio. That is why you may hear traffic from several different small airports on the advisory frequencies.