The filtering is useful for very long lists of entries, such as font lists.
The error displaying allows you to enter non-matching text (such as a font that is not installed on a PC, but is installed on another PC, or will soon be installed) and keep that text saved, as opposed to a standard combo-box which would require an exact match.
As shown in the example on the right, as you type, a drop-down list appears, containing the available entries that match the text typed so far.
The matching entries include all entries that contain the text typed, not only those entries that begin with that text, as shown in the last entry of the example on the right.
If the typed text does not correspond exactly to any of the available entries, then the text will appear in red, while the control has focus, as shown in the example on the left, below.
As soon as the focus moves to another control, then the non-matching text is shown as being incorrect by a red squiggly line (similar to the marking of spell checking) and a hint appears, as shown in the example above right.
Displaying the full list of available entries
Sometimes, you are not sure what the name looks like, of the option you are looking for. To see the full un-filtered list of available entries, delete all the text, and the full list of available entries will appear.