The "Time" field is great for recording preferences with regards to scheduling. If you have people who can only volunteer after 5 PM or opportunities that start on one day and end on another, this is the place where you can set up those preferences.
If you wish, you may set the default to the time the record was created by deselecting "No Default" and selecting the "Time Record Created" checkbox. Or you may enter your own default time.
Time can be set to use a 12 or 24 hour clock.