UV clarifiers are the key to clear water. They destroy single cell algae as it passes through the lamp giving clear water.
Each pond is different and positioned differently to the sun so may require a large or smaller lamp dependent on this. Where at all possible, go for the largest output within budget to ensure water clarity.
UV lamps do not kill blanket weed as the algae has to pass over the lamp in order for it to destroy the cells.
UV lamps in high wattages are proven to effectively kill some bacteria and parasites, although most pond size lamps have insufficient power to do this.
UV lamps also assist in breaking down chlorine, chloromines and other chemicals (hence most treatments advise to switch off the UV during treatment).
Under no circumstances look directly into a UV lamp. This is instantly dangerous and will cause irreparable damage to your eyes.