medics
To simpilfy, the PL tone is used as a sort of department identifier. It helps units for a specific agency hear calls that concern their coverage area. Calls with a differing PL tone never break squelch on their radios and are not heard. It is not essential to use when scanning.
However, there are times you may wish to use it. For example, in my area, Dallas county, fire frequency 154.400 is used both by Balch Springs & by Glenn Heights (which straddles the Dallas/Ellis county border). While they are currently migrating to an EDACS system, 154.400 has also been used by the Rockwall fire department. It was not uncommon to hear traffic for all three departments within a given time period. If I wanted to only hear Balch Springs on the frequency, I would enter their PL tone from the database when programming. If I wanted to hear any and all activivity on the frequency, regardless of department, I would leave that field blank. Not all scanners accept PL tone information as part of a channel setup.
Steve