From the WebCite website:
"Authors increasingly cite webpages in medical and scientific publications, which can "disappear" overnight. The problem of unstable webcitations has recently been recently referred to as an issue "calling for an immediate response" by publishers and authors.
WebCite® is an archiving system for webreferences (cited webpages and websites), which can be used by authors, editors, and publishers of scholarly papers and books, to ensure that cited webmaterial will remain available to readers in the future. If cited webreferences in journal articles, books etc. are not archived, future readers may encounter a "404 File Not Found" error when clicking on a cited URL.
A WebCite® reference is an archived webcitation, and rather than linking to the live website (which can and probably will disappear in the future), authors of scholarly works will link to the archived WebCite® copy on webcitation.org.
WebCite® is an entirely free service for authors who want to cite webmaterial..."