The "Browse..." button allows you to browse to a folder (using mouse input), whereas the edit box allows you to type in (or paste) a folder name (using keyboard input). Having the directory picker pop up as soon as you click the input box would cause 2 problems:
- You cannot copy-paste folder names any more
- You cannot type in folder names any more
- Users don't expect that clicking an edit box pops up a dialog.
That's why you need a button and an edit box.
Ps.: I assume that with "directory picker", the following dialog is meant:
This dialog does not allow you to type in a folder name as others mention.