The landing of the first stage in the Atlantic (a process that required decelerating it and bringing it to a hover just above the surface of the ocean before letting it fall in), is part of the resupply mission to the ISS. That is, once the first stage boosted its cargo towards the ISS, it then performed this test.
Too bad that they didn't try to return the first stage to land and then try to land it there but I understand their desire to do things one step at a time (it's safer this way also). I'm curious to know if this first stage had landing gear attached (maybe not because of the additional weight, drag). Also, in the future when they DO try to land it on land, where will they be aiming? If the flight profile of the first stage is mostly vertical then, without much fuel I guess they could return to Florida, otherwise would they be going for a Caribbean island? The Azores or Canary Islands? Africa? I'm sure they've got this figured out, I'm just curious.
Anyway, if they manage to recover the first stage by soft landing it without dunking it in salt water, it could REALLY drop the costs of space flight, even if they don't manage to reuse the 2nd stage (which they plan to do also). I remember reading that of the $20 million cost of a launch only about $500,000 was due to fuel, so this is a complete game changer. Even if the stage can only be reused a few times it'll make access to low earth orbit (the expensive part of space travel) much cheaper!
I only hope and pray that it works reliably and that the weight penalty is not too great! I thought they would have to use a lot more fuel to slow down and turn around but I guess they're using air resistance for the braking and the (now almost empty) booster is very light. Pretty unbelievable when you see a 10 story tall rocket turn around and land on a pillar of fire.