Combobox Question

I see that a Combobox allows you to type characters into the text field. What's the point of this?

I'm trying to find a way that if user types into text field it will stay and also become part of the list. Can this be done with out using a command button, menu item click, or any other control. I tried using Change but the problem here is I get one character at a time and if I do a AddItem then I get a list looking like this:

a
ab
abc
abcd
abcde
'
'
etc