If a site has been around for a while then it may need a fresh look, but if it functions well then a rebuild may not be necessary.

Redesign: If a site is working properly and has been constructed according to good web standards then there may be no need to build a new site from scratch. Colours, fonts, background and foreground images can usually be changed without affecting content. A certain amount of S.E.O. work may also be done, pages can be added or removed, and certain new functionality can be added, for example a map or slide show.
This is even more true of C.M.S. driven sites than traditional static sites, provided the system on which a site is build is sound and these are more often redesigned than rebuilt.