it's pretty much a way not to take up another USB port.
For example, lets say you have some pictures on your camera or maybe even a video, just take the SD Card out and put it in there. It'll read it faster and I think that some cards work faster than USB too.
So, it would make sense for Apple to include that, since they're very big on no clutter, no wires. And this is definitely a way to make your life easier by just putting the SD Card in there, rather than connecting a USB to your camera then to your computer and all that.
Also, maybe you have a phone or a camera that supports that, then you can take it out and quickly add music, pictures, videos, or whatever your want to it without having to go through the whole USB thing again.
And people would bitch and moan if they didn't put that feature in, which I'm sure that more than half of the people don't even use.