Even with the limited information collected using CGI, some possible uses for discussion sake could be:
a)as you already mentioned, to selectively deliver content based on the language used by the client, serving the page requested in the language reported by the browser, with a default language in case there is no page in the language the browser reports, or no language setting is returned by the user's browser.
b)Use special features only supported by certain browser versions or brands. By comparing the browser version the visitor is using, a page with heavy DHTML can be served knowing that it can be viewed correctly by that user. Likewise for certain CSS functions which only certain browsers support.
c)Using the IP address as a means of identifying certain users. For example, addresses starting with 24.*.*.* are coming from a broadband (@home cable) connection, so pages you wouldn't dream of serving to people on dial up become acceptable, if they are not from a broadband IP range then they see the low KB page.
It is also possible to identify AOL users in the same way (by IP address or the AOL designation in the browser's ID) and serve pages specifically targetted to that audience.
If you wanted to go through the trouble of doing it, you could identify ISP's by geographic location based on their IP address ranges, and serve a geographically targetted version of the page being requested by a user from a certain geographic region.
d)Retrieve the keywords from a referrer and serve keyword specific versions of a page requested by a user based on their search engine query.