They probably just needed some sort of counter-mass or some other similar element and figured they'd turn it into a PR thing. Similar to how New Horizons ended

They probably just needed some sort of counter-mass or some other similar element and figured they'd turn it into a PR thing. Similar to how New Horizons ended up carrying some remains of the person who discovered Pluto, or IIRC parts with names on Perseverance of people who worked on the rover.