Band conditions are continuously changing and having a predictive type map allows ham radio operators the ability to visually see what they are hearing.  For example, the other night I heard a call on a local 440 repeater from a ham radio operator in Western New York over 230 miles away.  He was running 45 watts of power, but that was pretty good for a 70cm wavelength to reach that far.  To verify what I was hearing, I pulled up the VHF propagation map and noticed that we had a large skip anomaly happening so I knew the radio bands were open for great distance communication (we call this DX communication).

The following map uses APRS data to show visually some potential band openings coutesy of the Mountain Lake K-12 School District in Minnesota.