Jewelry, like clothing, is a means for a woman to express herself, celebrate her beauty and enhance her appearance. Wearing jewelry is something that is as old as the ages, but generally the personal adornment of the female is an expression of her individuality, like your choice of an outfit is your expression of who you are.
If you are a man and you are asking this question because Valentine's Day is getting near, the reason women like to receive jewelry as gifts is that it makes them feel special, since you spent a decent portion to acquire them something worthy of wearing, it enhances a woman's sense of worth.
If you are a man and you have a girlfriend, you should periodically give her good pieces of jewelry if she likes jewelry, to show she is valuable to you and that you care significantly enough to make it evident. You will, in the course of your life, receive back infinitely more than you give in this arena by doing so.
Having a significant other and NOT buying them decent pieces of jewelry periodically may make your woman size up someone who sees her as "worthy of diamonds and pearls." If you get my drift.
What do I mean by "good pieces of jewelry"? Something with diamonds in it. Something that looks like it is worth something valuable, if you want to make her feel that way. Nowadays, with the sales at the big chain jewelers around the holidays, acquiring diamonds for your lady shouldn't put you in the poohouse. You don't need to get her the Hope Diamond, but nothing says you care like diamonds...just make sure you care. There is nothing worse than receiving a gift like that, one that should say "I treasure you," when you don't...don't give it out of guilt or absence, but out of caring.
For eons, woman have enhanced their social appearance of worth through jewelry. Someone who has a solid sense of self-worth will wear jewelry when no one is around.
Jewelry makes you feel beautiful.
A ring for engagement and the size of the diamond is often the measure, on display to the world on the womans hand, of the level to which you value her. Shallow, yes. Social status obsessed, yes.
Realize that these have been Traditions for many millennium.
Jewelry enhances value. Period. Gorgeous, it also enhances beauty. Bold, it is a way for a woman to literally "declare" herself to the world, as many in fashions elite creators realm - Coco Chanel, Babe Paley, etc - were known for their bold, making-a-statement jewelry.
Finally, jewelry is wearable art. You can't hang a Picasso around your neck, but these diminutive delights, you can.
