Why would you ever want to upsample a photo?

You at least retain control of the process if you handle the upsample.  Yes it depends on the particular image, and there are numerous means to do the upsample.  To my mind, the recently introduced Preserve Details 2 is a clear improvement on the original version, and the best process to increase pixel size currently available.

adamh88  wrote Say you have a 5 x 10 photo at 200 PPI and you are sending it to an offset printer so you upsample it to 300 PPI. Is the printed photo really going to look better upsampled?

No, you're absolutely right, it will not look better. Usually much worse because of artifacts introduced by the resampling.

Printing at 200ppi will usually turn out just fine. Contrary to what people believe, it will not be noticeably less sharp. It just means you may be able to make out individual pixels (which by definition you cannot at 300).

The line screen is still 150 lpi. That's the effective resolution.