Cell phones and palm computers may have only a small numeric keypad where a short numeric password is the only practical form of secure input. These environments may not provide secure long-term storage appropriate for private keys or password verifiers. The data can be promptly erased from the device to remove any long-term threat. These environments also are hostile to long-term storage of persistent keys. The ability to use a small keypad for remote banking, without requiring trusted third parties, seems particularly attractive, and impossible to achieve by other methods.
Diskless workstations are another class of device where it is inconvenient to store keys locally.
The user's device or system can be generic, and not pre-configured to serve a specific user. Alternative approaches using a persistent public-key must embed a key designating some highly trusted third party, such as a global Certificate Authority. This may be inconvenient, risky, or introduce unnecessary recurring expense. The concept of bootstrapping a new secure system is broad, and is best illustrated with a common case. Somewhere during installation you re prompted to enter a name and password to let you join the secure corporate network.