The extension tube serves to increase the distance between the lens and the sensor. This allows the lens to focus closer and, therefore, increase magnification, so you can use almost any lens for close-up photography. As with a macro lens, the longer the focal length, the greater the working distance you can achieve.

What is the disadvantage of extension tubes?

The only disadvantage of extension tubes is that there is some light loss. Adding an extension tube increases the effective aperture of the camera lens, which means you need to use either a longer shutter speed or higher ISO to compensate for the loss of light.

Do extension tubes affect image quality?

Extension tubes work by shifting the minimum focus distance towards the camera. With the lens able to form images closer in, the subject is effectively magnified. Since the tubes don't have any glass- they just add some extra space- their use does not degrade image quality.