What are the advantages and disadvantages of UV or ultraviolet light?
The disadvantage are that it can cause skin cancer if you are exposed to sufficient amounts of UV light (in particular, UV-B and UV-C light).
See the Related Questions link to the left of this answer for more information about that: "How do UV-B rays affect people?"
One of the beneficial uses of UV light are that it can be used to kill bacteria and other microbes. It is commonly used to purify water for instance or clean things. UV light can also be used for a variety of other of application, such as black lights (which just look cool!). Also many compounds fluoresce (they emit light) when exposed to UV light. This is how forensic scientists can find traces of blood and other bodily fluids at crime scenes for instance. Finally exposure to some UV-B light causes the body to make Vitamin D, an important nutrient. While some exposure is important too much will cause sunburns and possibly cancer.