A perfect example of what you'd use the reader for is if you wanted to put MP3 files onto your phone. The quickest way is to put the microSD card into the SD adapter (what you're calling the "reader" and put this SD "card" into the PC - either directly if it supports it, or with a USB adapter. Then you can see the microSD card as a drive on the PC and copy all the MP3s to it.

This is generally much quicker than putting the microSD card into the phone and connecting the phone to the PC with USB.