The color picker can be used to transfer the color of the pixel on the current layer at the mouse cursor's position to either the primary or the secondary color. If you left click, the primary color will be changed. Right clicking will change the secondary color.
It can be useful to use the Color Picker tool with the grid mode enabled while you are zoomed in on an image, in order to do per-pixel selection and fine-tuning.
Selecting a portion of the apple changes our primary color to our selection