You can set any input radio or checkbox when they are created.
For example, suppose you're going to ask a question you're pretty sure you know the answer to, but can't be completely certain.
The HTML to produce the above is:
Are you awake at the moment? <INPUT type="radio" name="awake" value="Y" checked>Yes <INPUT type="radio" name="awake" value="N">No
Obviously, you will want to use CHECKED on only one option in a radio-button logical input, as shown above, but multiple checkboxes in a given logical input could be set as CHECKED.
Text inputs and buttons to select are all very nice, but they clutter up the screen and get a little boring after a while.