The user needs to enter an item into a text box which could ambiguous or hard to remember and therefore has the potential to be mis-typed.
Auto Complete is under-utilized in most web sites and web applications. Most text fields could benefit from the addition of this feature. When done correctly, it lightens the load of the user both mentally and physically.
We are currently planning to provide this behavior in an upcoming release of the Yahoo! User Interface Library.
The pattern can be found at: Auto Complete.