Hard solder does not pit as medium and easy do. See the photos below. I teach my students they do not have time to dork around using three solders. In the time it takes to decide which one to use where, they can have it soldered and finished. If a piece gets ran over by a car, the poor slob that must repair it, will not have fall completely apart as he, or she, heats it up.
Hard solder makes stronger solder joints.
If you want to know why joint the Silversmithing List and ask. The answer is simple and will makes sense to you. See test below. To test the strength of the three grades of solders, I performed the following test in one of my silversmithing classes for my students. I was a little shocked at the results.