Simple, when the native resolution output of your camera is smaller than the application you need it for. The ideal? No uprezzing (if you have to I like Genuine Fractals). Uprezzing introduces artifacts and requires additional sharpening to get the file to appear sharp. All of which may be acceptable, I have done it for years, but not ideal. For example for magazines, I need 300 DPI for a given reproduction size. BUT in terms of submitting, when I do not know how they will be used, I need to submit for the largest possible usage which is a horizontal double page spread, approx. 10"x16". See the Su Casa cover example in the "Getting Published-Magazine Covers" post.
That is a typical magazine usage with some generous cropping to get the composition right for their purposes. That image was made with a 5D and the file was delivered with a 20% uprezz in Genuine Fractals (with a 21MP 5DII that would be unnecessary saving me some comuter time on all magazine shoots).