Ben Wah balls are often called other things like Benoit balls, Orgasm balls or Geisha balls. If they are larger and made of plastic they are called duotone balls. There is documentation that they have been used all over the world for centuries. For women, they have been used by cultures from China to Egypt.
Regardless of what you call them or how you heard about them, they are generally used for 2 reasons- either as a subtle stimulant or to strengthen the pelvic floor. Some believe that wearing them and rocking in a chair is a turn-on. Others say just inserting them and wearing them throughout the day can be stimulating. For pelvic floor strength a practitioner may instruct you to put them in and keep them in for at least 15 minutes. The smaller ones require quite strong pelvic floor muscles. If those muscles get stronger it can help with urinary incontinence.
For someone beginning to explore them I would suggest the larger duotone balls, made in plastic and generally attached with a string that makes pulling them out very easy. For the more experienced user, the metal or glass beads are fine. They are sometimes elaborately made with flowers or designs within the balls. They can also be used internally during sex or with a clitoral vibrator.
Ben Wah balls are not very commonly used now by women. I think the reason is because there are far easier ways to strengthen your pelvic floor and to find subtle ways to be stimulated throughout the day. That being said, they can be fun. If you want to give them a try start with the larger ones and graduate to the smaller ones. A strong pelvic floor is good for many reasons. It can make sex for you and your partner more enjoyable, and it can help with your over all pelvic floor health.