The availability of dissolved oxygen in water, essential for healthy roots, can be enhanced by bubbling air through the nutrient. It can also help to lower the water temperature in the summer and reduce the risk of disease.
Water pumps
Here's a wide spread of pumps to suit every need. Micro and Maxijet pumps have proven to be exceptionally reliable over the years. Growers often use the smaller pumps for re-circulating and mixing their nutrient tanks. Also, you'll find the laborious job of draining a tank a lot quicker if you use a large pump such as the Aquaking range.
Air Stones
Ceramic air stones offer the smallest size bubbles with the added feature of not being affected by the acidic nature of nutrient solutions - just scrub them clean. Use the blue balls for a cheap and cheerful solution to your bubble requirements.