[LAU] Rolling off high frequencies when mastering?

Kind of a general mastering question, but obligatory Linux screenshots of JAPA are included, I promise. I've noticed with some professional cd's/oggs/mp3s I have, the high end is rolled off at around 20Khz. Some roll off hard core: http://storage.restivo.org.s3.amazonaws.com/rolloffs/hardrolloff.png Some have a softer, gentler rolloff, but they still roll everything off: http://storage.restivo.org.s3.amazonaws.com/rolloffs/softerrolloff.png My own mixes, however, don't do that. http://storage.restivo.org.s3.amazonaws.com/rolloffs/norolloff2.png Now, the examples in question are from the 1980's and 1990's, because that's the last time I actually bought a CD. The question is: why do they roll off like that, and is there some reason I should do it in this day and age? The LADSPA GLAME Lowpass filter in 4-pole mode seems to do the trick, it's maximum freq is like 19.5Khz though. But should I? -ken