I'm a sucker for infomercials and don't you know I called right away to get my own set of Aqua Globes and they're working great. You'll get a set of four in different colors and they'll offer you an opportunity to purchase additional sets at a discount or larger sizes. I now wish I had opted for both.
Purpose?? Not only are they decorative but you fill them with water and the globe shape holds the water in and the soil slowly absorbs it as needed. You can't have too many of these!