I have 30 ignored tags on SO. When I load a page it takes several seconds for the client-side code to remove questions (during which time the browser is frozen). Sometimes this results in far fewer questions per page than normal (16 instead of 30 when I checked just now). If the filtering was done on the server side, the "frozen browser" effect would be avoided and the number of questions on the page would be correct.
Note: I couldn't find a feature request for this on meta already. If there is one, please post a comment with the link and vote to close as a dupe.