Getting closer has the effect of magnifying your subject (making it appear larger in the viewfinder and in your pictures). They are exceptionally useful for macro photography, enabling you to convert almost any lens into a macro lens at a fraction of the cost while maintaining its original optical quality.
Please note:
When using extension tubes the lens will not focus to infinity. The focus range will be greatly limited to a very close focusing distance.
There is light fall off when using any extension tube, sometimes the e quivalent of 3 f-stops of light is lost when using multiple extension tubes together. This light lost can affect the camera's ability to auto focus. Manual focusing is recommended should the lens won't lock onto the subject.
Depending on available light and due to a very shallow depth of field manual focusing may be required if the lens AF will not lock focus.