what the difference would be between using it and just using a single exposure at a slow speed

The difference is that you can take several completely different exposures all in the same frame. There are lots of ways you can use it.

For example, you could take a shot of a landscape and then a shot of a person against a dark background. The effect you'd get would a semi-transparent person layered over the landscape.

Another, perhaps more likely example: You can take several exposures of a single person in different poses in different parts of the frame. This works best with a black background, so that nothing in the background bleeds through the images.

A more common variation on the same theme is to fire a strobe multiple times during a single long exposure to freeze motion at several points.

This kind of thing was much more common in the film days. With digital, it's so easy to combine images in different ways after the fact that it's not necessary to do it in camera. It's still fun, though, and might get your creative juices flowing.