1. The default file chooser on windows is horrible.

2. With multiple monitors, multiple desktops, etc, quite often people will have their working files open anyway.

3. You can drag a picture straight from another open browser tab so it never needs to touch your browser.

4. Most sites give a choice of either method.

5. It's a fairly intuitive action. I know that I caught older family members struggling when they used to drag images and then the image would display in their browser. At this point they thought the photo was online and would be confused when they couldn't then find it on the site when they came back to it later.