From the sound of it, it was going out to replace a plane that needed maintenance. Which is a thing that happens; just a couple weeks ago, IIRC, Delta had to pull a 747 out of a "boneyard" storage facility and fly it to Seoul to replace one that took a bunch of damage (mostly hailstones) from a thunderstorm.
Also, schedules are built around keeping the plane in the air as much of the time as possible, which sometimes means there's a mostly-empty "positioning" flight to get a plane to where it needs to be to carry a full load. I know with US Airways, for example, you can sometimes randomly snag a seat on a widebody international plane between Philadelphia and Charlotte (usually coming out of maintenance and onto transatlantic service, or switching which hub the plane operates from), and on Delta you can occasionally fly a 747 domestically between Atlanta and Detroit (to get the 747 in position to fly from Detroit to destinations in Asia).
I wanted to try first class on the 777 when it first came out, so I just connected in DFW for a PHX-ORD trip, and did the DFW-ORD segment on a 777.
As it turns out, domestic flights are GREAT in big aircraft. (Though I do like the smaller A321Ts too. International-class accommodations, but a really small feeling, like a private plane.)
Also, schedules are built around keeping the plane in the air as much of the time as possible, which sometimes means there's a mostly-empty "positioning" flight to get a plane to where it needs to be to carry a full load. I know with US Airways, for example, you can sometimes randomly snag a seat on a widebody international plane between Philadelphia and Charlotte (usually coming out of maintenance and onto transatlantic service, or switching which hub the plane operates from), and on Delta you can occasionally fly a 747 domestically between Atlanta and Detroit (to get the 747 in position to fly from Detroit to destinations in Asia).