I use grids a good deal of the time. They absolutely control directionality and limit spill to other areas of your set or subject area. There are fabric grids used in softboxes and octaboxes and there are spot grids (sometimes called honeycomb) that are attached to the reflector of a strobe.
If, for example, you have a small space and your subject is close to the background, and you're using a medium softbox, the spill and falloff are going to pour light on to your background. Sometimes that's what you want but sometimes it absolutely is not what you want. By using a grid, you can control the directionality and direct the modifier diagonally across your set or subject area, and in doing so you can effectively keep light from reaching the background. Without the grid you would have to work very hard to control the light from that modifier and the only way you could keep light from reaching the background would be to distance the modifier and subject area from the background,. which we're already established that you can't do because of space limitations.
There are many uses for grids. They can be used on hair lights, accents and kickers, and on background lights to create a tighter spot of light than you might get from a standard wide angle reflector.