The Degauss option de-magnetizes your monitor. With CRT monitors static tends to build up on the screen slowly causing a charge to build up which may warp the display a bit. Degaussing the monitor discharges the static charge. There isn't really any rule about when to degauss. I just do it once a month or when I get bored. Sometimes you will see a distortion in the corner of your monitor which is caused by a magnetic field. Degaussing may help resolve the problem if the magnetic field is not too strong.
Steven S.
MCSA
A+, Network+, Server+, i-Net+