Drivercleaner was used when uninstalling graphics card drivers and/or changing from Nvidia to ATI or vice versa. Its CAB cleaning tool removed residual .SYS files, support DLLs, language resources and pretty much anything else not needed.
As for it being necessary...I haven't used it for ages, but it is a good application. Plus the only way that you can get it now is to pay for the Professional version
. The free versions that are kicking about still would be too old really for anything