So what's the big deal with landing rocket boosters? Until now, boosters were essentially "use and throw" carriers of payload. They were huge monetary investments that were being hauled and destroyed in space. Bringing them back would greatly reduce the cost of sending stuff into space and also save gallons of fuel. It would also reduce the time and money spent in building new rocket booster for every space mission. Landing boosters could actually catalyse space exploration.
But of course, landing a tube filled with explosive fuel upright and in a controlled manner is a big technological challenge. These rocket boosters can be extremely dangerous if they ‘fall’ on earth. In fact, landing on earth doesn’t make sense at all, since rockets have a curved trajectory: once the payload is released into space, the rocket is usually above the ocean. Using fuel again to thrust the booster back towards land isn’t technically viable. Rather, using the remaining fuel to perform a controlled landing on the ocean makes a lot of sense. That’s the idea behind SpaceX’s landing on a barge.