Playing with only one die is the least interesting case, but it can without doubt be useful:
You can achieve any result from 15 to 20 (that is 6 possible results).
You have a chance of 1/6 for each of these results, just as you have when rolling the traditional way.
This can be used in several new (not yet invented) types of games, or in modifications of existing games.
For instance, you might imagine a game in which you move your token after rolling a die, and sometimes (in specific situations) you get to move after an inverted die roll (which is a significantly larger number). Maybe even games like Ludo might be made stimulating in the future.