If you are layering pendants, bail size matters. Pieces designed for it typically have wider openings specifically to accommodate two chains at once without forcing anything through.
Built For Ease
Mechanically attached and hinged bails can be replaced or upgraded by a jeweler, which matters when the original bail is incompatible with a preferred chain or has sustained wear at the attachment point. Soldered bails are a different situation, requiring more involved work to replace.
Visually Speaking
Other pendants have one audience. A locket has two. Public on the outside. Completely private on the inside. It's the only pendant format that keeps a secret.