To look good, because they like the look of them, make it known they have the money to afford it or make a statement about their religion/ideology/social status, for sentimental or religious reasons, to remind them of something, because it has some practical use (like it has a clock, or if you include eye-glasses), draw attention to a part of their anatomy(often breasts, or ears) etc.
I don't normally wear jewellery, but I have a lot of family jewelry or stuff I think looks shiny that sits in a box. There are some small things, rings and necklaces I put on as a part of my attire for formal occasions, but nothing too big.
Oh, and a small golden spider I wear on my fedora, because I like spiders. All of the jewellery I wear portray animals, I just like the idea of a lizard or a small bird sitting on my clothing, or a serpent around my finger, I guess.