Once in a while we need to use the brush to paint with a certain color, which already present in the image. For example, we want to get rid off some of small imperfections on a smooth surface. If the surface has a texture that I want to preserve, then I would use the Healing Brush or the Stamp tool. But in a simple case (there is no texture) I would have to first switch to Eyedropper and back to brush. The more colors I have to switch in between, the more clicks and movements I have to make. However the developers of Photoshop provided us with a nice feature - hold Alt key while using the Brush tool and you instantly switched into Eyedropper tool. Then click on image where the color you need to reuse is and it becomes your foreground color.