Because the open side faces a specific direction, you're sure the charm will always hang facing forward when you attach it to your earrings or necklace. If you attach a chain charm to your earrings, it will face the side.

Necklace charms have a larger jump ring, so you can be sure they won't break while putting on your necklace. Necklace charms always fit your earrings, but because of the more subtle jump ring, earring charms aren't suitable for necklaces.

Does the charm not have a front or side (e.g., colored charms)? Then choose a chain charm. This way, you're sure it will fit around your earrings and necklace!