Read more at Adorama’s 100 tips in 100 days.
A higher ISO setting means the camera is more sensitive to light and will result in the camera selecting a faster shutter speed and/or a smaller aperture. On cameras which don’t allow manual control of shutter speed and/or aperture, the only way to change either may be via changing the ISO.
So, for example, if you want a higher shutter speed to stop action, one way to get it could be to set the ISO to a higher value.
Similarly, if you want to maximize the depth of field for macro work, you need a small aperture. This can often be achieved on all-auto cameras by choosing the a high ISO–say, ISO 400 instead of ISO 100.