What is Ultra-Violet used for? Why?

When UV is Absorbed, and Visible Light is Given Off, its called Fluorescence, a Property -This can lead to Fluorescence Lights, which are Very Efficient Lights for a Long Period of Time -Security Pens also uses UV, and it can Mark Whose it is when viewed under UV Light -UV can also damage the DNA in Skin Cells, so being out for a While can Damage the Skin Cells