The mechanisms by which people interact with computers are very limited - typically using a keyboard, mouse, and monitor.
User information is principally a local phenomenon.
If you log in from a different machine, your preferences, data, and applications are not accessible. Users must directly act on information, rather than setting intelligent preferences that then automatically act on the users behalf. Data for the same user across different applications and sites is difficult or impossible to automatically integrate into a single, coherent view for the user. Users who want to work from home or on the move are constrained in the access to applications and data that reside only on their machines at work, creating a barrier to additional productivity. Web sites are isolated islands and cannot communicate with each other in on a user's behalf in any meaningful way or without significant additional development.