Known as the Keyring tool, this is really a way to provide an attachment point for the knife. The attachment point is suitable for any Split-ring, such as a key-ring, and the small split-ring usually supplied with the knives provides a larger attachment point that is suitable for a lanyard or simply connecting to a keyring.
The Keyring can been seen as a replacement for, or alternative to, the Shackle or Bail attachment mechanism that was used mostly in the past. Victorinox also used a Hollow Rivet as an attachment mechanism between 1964 and 1992.
Some knives use a hollow rivet in the construction of the knife and expose this through the scales. This allows a lanyard to be attached by simply threading it through the rivet.
The knives with a USB Flash Drive have a different split-ring attachment point on the Flash Drive cover/release.