Extension tubes a great way to focus much closer to small subjects such as an insect or flowers without the expense and weight of buying and carrying a macro lens. One extension tube takes up only as much room as a couple filters in a camera bag. Keep one or more in your camera bag at all times and you will always be ready for that macro shot.
They are designed to enable a lens to focus closer than its normal set minimum focusing distance. Getting closer has the effect of magnifying your subject (making it appear larger in the viewfinder and in your pictures). Extension tubes are exceptionally useful for macro photography, enabling you to instantly convert almost any lens into a macro lens at a fraction of the cost while maintaining its original optical quality.