It allows you to type a value that does not exist in the list.
For example, on a web to lead form we use the field called lead source. In the placeholder we have the question how did you find out about us. Since the drop-down arrow is not very visible, a customer can type their own notes into the field instead of selecting a value from the drop-down.
This should either clear the field, or pop up a message.
There has also been a complaint about needing to go all the way over to the right side of the field to click the drop-down.