But most launches don't need it....
"......Just like the v1.0, the Falcon v1.1 launcher provides engine-out capability for a large portion of its first stage flight. All nine engines are ignited on the ground, about three seconds before launch. All must reach operational conditions and liftoff thrust for the launch release command to be issued.......
The first stage has a primary burn time that varies depending on the mission design. Flights using the full performance of the first stage without propulsive return maneuvers burn the first stage for up to 185 seconds while missions that include a first stage return require the stage to shut down its engines after 155 to 165 seconds of powered flight to leave sufficient propellants for the descent to a landing site.........
The re-usable version of Falcon 9 is known as F9R which itself does not represent a fully different launcher and is more of an add-on to the v1.1 version in the form of the Nitrogen Cold Gas Attitude Control System, the four deployable landing legs and four grid fins used for three-axis control during atmospheric flight, especially during non-propulsive flight phases.....
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So for most missions, its just some additional fuel. The aim, of course, is to reduce the number of missions where insufficient fuel would be left to enable a return and hence loss of the first stage. That's where the barge comes in rather than a return to the Cape.
The return to the Cape needs enough fuel to flip over, stop, then reverse speed and head back. With the barge positioned along the route the first stage can follow pretty much a ballistic curve from its separation point only need enough fuel for small course adjustments and the final deceleration to land.