This is something that doesn't matter that often, but in the right situation can be very useful. For example, I just completed a task in which I needed to apply a component that plays appropriate selection/activation sounds for a button. This component requires that I set an audio mixer channel, and fill in lists of which sounds to play. Since the menu has dozens of button objects, many of which do not have a prefab, these assignments would need to be manually done dozens of times.
This is obviously a chore, and beyond that there's a lot of room to accidentally miss a couple of fields as I went through and did them all. Using default properties, however, I could avoid those problems. I can simply select the script file, fill in the defaults for those fields, then whenever I added the component the standard sounds and channel were filled in automatically.
For reference, when selecting a script file look for a section at the top of the inspector panel, like so:
