TLDR: Never, unless you are not making any progress (losing progress)
A progress-bar in UX has one simple feature, show progress to a user. Progress is a forward motion/movement. This is I especially true from a psychological point of view.
From the Cambridge dictionary:
Progress - movement to an improved or more developed state, or to a forward position. https://dictionary.cambridge.org/dictionary/english/progress
In the context of a progress-bar this would mean that the bar needs to have a forward motion, or in other words a left to right filling motion.
Reading direction: the direction in which a progress bar fills, should in my eyes logically depend on the natural reading direction for a specific language. For RTL languages the bar should fill from the right to the left. However most RTL interfaces I've seen do strange enough not reverse the progress-bar. I'm not sure why this is the case, do people simply forget the progress-bar? Or is my theory based on nonsense?
Don't make progress go backwards
Quite some progress-bars have a forward filling motion even though stuff is being removed. Take for example the file removal dialog in Windows:
The progress bar here could have been reversed, but that would communicate a completely different message to the user: "The disk getting emptier". But in a forward filling motion it would communicate the correct message: "The progress of removing the files".
An easier example is emptying a bucket of water. You can show the progress in two ways:
- How much water did we already drain from the bucket? (from 0 to 100)
- How much water is left in the bucket? (from 100 to 0)
From a psychological perspective the forward motion from 0 to 100 is preferable.
If you look at progress-bars on different crowdfunding websites (take Kickstarter for example) they always seem to fill up from 0 to some number (and from the left to right) but can easily be reverted:
500$ of 2500$ funded
[===== ]
These websites could however have used a different type:
2000$ of 2500$ needed
[=================== ]
The first type fills from the left to the right, the second model empties from the right to the left. The first progress bar is used for a reason, it shows the progress in a forward motion. The backward progress is counter-intuitive.
When can you have backwards progress?
Almost never, unless you are for some reason losing progress. A good example of a backwards going bar is the health-bar in a game (as mentioned in the comments). A health bar always depletes as you lose life. Notice however that a health bar is for a good reason not called a "progress bar". A health bar does not communicate progress to the user, instead it communicates something negative.
Conclusion
Progress is forward motion, backwards movements is counter-intuitive and from a psychological point of view probably not the best choice. Forward motion is positive, backward motion is negative. Because of this I suggest you use a filling type progress-bar.
Note: I can't currently find a good source to back this answer up with. But I do have an interesting paper that researched different types of forward going progress (linear progress vs accelerating progress etc.) : Rethinking the Progress Bar
