I have had a bunch of times when you need to use a hard drive designed for a Laptop in a desktop system, say for formatting, or copying massive amounts of files. Laptop drives are by spec. 2.5inch wide, while desktop drives are 3.5 inch wide. This isn’t a problem for the average user because we have these nice 3.5 to 2.5inch adapters, however what if you wanted to put a full scale drive into a laptop? You’re screwed. Of course, in most cases you couldn’t even fit the full drive into the laptop just due to it’s actual size, not just the different adapter.
Read more about my build here:
http://www.instructables.com/id/Using-a-full-sized-HDD-in-a-Laptop/