Wayback machine can be very interesting and occasionally useful for finding informsation which used to be on a site which no longer exists. When Mudcat is down, you can read old threads from specific dates which have been archived there, for instance.
Sometimes it does not help, though. Some times the site owners have requested that the archive doesn't store their data. Sometimes there ate copyright disputes, so stuff cannot be kept. Sometimes, particularly with web sites where a blue clicky does not link directly to another web page but instead runs a script which does something, or refers you to a location which hasn't got an equivalent within the archive.
That said, the Internet Archive also contains stuff which is NOT the Wayback Machine. You can download Grateful Dead concert and other bands who have given the archive permission to freely distribute their work (there's a lot of bluegrass, punk rock and weird stuff).
Quack!
GtD.