The llTextbox function is frequently used to enter parameters as values or text. It could be useful to prefill the text field with the previous parameter especially if it's a long text.
Example, this previous sentence was entered: "The quick brown fox jumps over the lazy cat." and need to be fixed. Currently, for edit it, I need to put the sentence as message in llTextbox. Then, copy it from the box label and paste it in the field. With the llFilledTextbox, the field would be prefilled with the sentence. Also, a text size extended to 512 characters would be welcome. To finish, the field needs to take the focus when the box is showed.