Up until this flight of Falcon Heavy, officially, SpaceX could not fully deliver a satellite to [nearly circular] GEO (Geosynchronous Earth Orbit), but only to a [highly elliptical] GTO (Geosync Transfer Orbit) that expects the payload to circularize its own orbit once at the appropriate altitude.
This consumes fuel, and fuel for station keeping is one of the major determinants of satellite lifespan. Time is money, therefore the fuel required to get from GTO to GEO itself costs money.
Now many GEO operators are ok with this added cost, when they get a reduced launch cost overall, but it is a real factor.
The Department of Defense (DoD) has 8 reference orbits, of which direct insertion to GEO is one of them, and in order to compete with the Delta 4 Heavy for such launches, SpaceX needs to demonstrate that they can boost a payload to that orbit.
Getting to GEO direct, requires a coast phase after the initial orbital insertion, (as I've discussed in more detail in this answer). That is, first and second stage deliver the payload to orbit, and then a few hours later a second burn is initiated that finalizes the GEO orbit. One of the problems with the coast phase is radiation, another is the cold and lines freezing (which happened before to a Falcon 9 upper stage test).
The first Falcon Heavy flight was a demonstration flight testing many aspects. This can be seen in the loss of the center core due to lack of TEA-TEB ignitor fluid. They knew how much it takes to reignite the needed engines, and were clearly pushing the lower end of required ignitor and estimated low.
Thus this was just one more way to get some value out of a test flight. The payload was a media extraveganza and hilarious. The mission was a success. The second stage was able to test GEO orbit insertion. The center core test did not succeed, but it sets a lower level and now they have more information, that they did not have before.