Just landing on a larger target than their barge would be easier. The April landing probably would have succeeded if they hadn't had to make a big horizontal move just before landing to get into position over the barge. Once they get the barge landing working once, they plan to switch to landings on a landing pad, which can be made much bigger than the barge.
The horizontal correction at the end was part of the plan. I suspect the purpose is to come in at an angle so that the direction of the final correction is approximately known, and they can correct only by varying the amount of thrust, not the direction of the thrust (which is presumably slower).
Vs. coming in right on target - you'll still have small corrections to make at the end, but you have to gimbal the thruster in a random direction.
They come in at an angle so that the rocket is not over the barge for long, that way if it fails higher up it doesn't fall onto the barge with a lot of kinetic energy.
