UV light is able to effectively eliminate mould because the electromagnetic radiation that it produces is able to work as a disinfectant and can penetrate through organisms. When this light is placed in a lamp and sent out in short wavelengths, these wavelengths will eliminate most of the mould cells and spores while making sure that the remainder of the cells are unable to reproduce until they die out. Because of how quickly mould can spread, using ultraviolet light can be very effective at keeping mould from spreading as well as eliminating it at the same time.
Manually getting rid of mould is ineffective because even one missed spot of mould can reproduce and cause mould to spread at the same rate that it was before you cleaned the area. When using ultraviolet light, the entire area that the light is spread over should be rid of mould in a short period of time. When the ultraviolet rays come across mould cells, they damage the nucleic acids that are a part of the mould.