The above link will take you to a page with various options on downloading several different Android emulators on your PC. I've been using Bluestacks for a couple of weeks now and it's satisfactory. I plan on trying a couple of the other ones on to see which works best.
Why an emulator on your PC instead of just using your phone?
Many reasons, but sometimes it's just convenient to use apps directly on your PC, instead of always having to hover over your phone. (Great for work)