The military places great significance on a legacy of honor and duty; every Marine honors the sacrifices and victories at Iwo Jima and Tripoli. The Army airborne divisions celebrate their jumps into France during World War II. Every unit that landed on the beaches at Normandy sacrificed a great deal in order to achieve their toehold in France. The Navy remembers the victory at Midway, and the battles at Coral Sea.
Due to these sacrifices, these great battles were won. The ribbons worn by current members of those units are not just wearing fancy eye candy to impress onlookers. They are honoring a great tradition of valor and sacrifice borne by their forefathers, with the hope that those current members will have the strength to uphold that tradition when needed.
On the Army dress uniform, individual awards are worn on the left side of the coat. These are your ribbons recognizing service, achievements, campaign ribbons, and individual awards for valor (including the Purple Heart) or combat (Combat Infantry Badge, Combat Jump wings, etc.).
The ribbons worn on the right side of the coat are those ribbons honoring the legacy of the unit to which you are assigned. As long as you are assigned to the unit, you wear its ribbons in honor of that unit's legacy. Once you transfer out of that unit, you remove those ribbons, UNLESS you were a member of that unit at the time it was awarded its citation. In that case, you wear that ribbon permanently.
Thank you for your kind inquiry into the way we demonstrate our respect for those who have paved the way for our freedoms.