All the better ones had that button, usually named "degauss". My 22" flat CRT (it was flat on the front, like LCDs today, which was a great feature at the time, but made the thing extremely expensive and extremely heavy) had a degauss function that was apparently so powerful it visibly distorted the image on other monitors right next to it when I degaussed mine. I made a joke of doing that in the middle of games on LAN parties ;-) didn't work repeatedly though, after degaussing once I had to wait for some time until degaussing again would actually do something.