PL, also called CTCSS, is a way of separating transmissions and/or mobile units sharing the same frequenct. Another system in common use is called DCS.
Using a PL/CTCSS tone or DCS code allows the user of the system to exclude receptions transmissions on the frequency that are not using the same tone or code. For the scanner listener, you can use the tone to help elimnate confusion.
An example of this is the frequency 154.400 in the Dallas County area. It's used by Glenn Heights FD, about 15 miles to my south, and Balch Springs FD, a little farther to my east. Until a year or two ago, it was also used by Rockwall FD, about 30 miles northeast, but definitely in range. So when you heard a transmission on the frequency, who was it? Unless you had a map handy, recognized a major street, something distinctive about the dispatcher's voice or style, unit numbers, whatever, you might not know for sure. Now, with separate PL tones, they can identified without confusion. I have each city entered in a channel, text tagged, with their unique PL tone.
The down side is that if you also had another agency (or mobile units) on the frequency that did not use a PL or DCS, you'll never hear them.
In some cases, using a PL or DCS on a frequency that is prone to interference from another electronic device may allow you to use a lower squelch setting and still avoid the annoyance.YMMV.