The QTH locator field is important when you intend to make VHF/UHF QSO's and you have a locator field in the log.
The main xlog window has a button next to the callsign entry. When clicked it will
lookup the callsign with your internet browser on your preferred server.
It takes the form http://www.qrz.com/<call>, where <call> will be replaced
with the callsign.
The use of the tilde character is explained in the "Keyer window" section.