All of the powder flasks I've seen serve to hold and measure powder.
Some of them, usually the old original ones, have adjustable spouts and most of the modern reproductions use a removable spout.
This photo from Track of the Wolf is an example of a Italian made flask that duplicates an old original flask.
The brass spout unscrews from the top of the flask and it is possible to buy replacement spouts of different lengths, giving different amounts of powder measured out. I have 10 of them.
To use one of these flasks to measure out a load of powder, I grab it in my right hand. Then, I put my index finger over the end of the spout, rotate my hand so the spout is pointed downward and then push the lever with my thumb to open the gate. A few shakes of the hand convinces the powder in the flask to fill the spout. I then relax my thumb pressure, allowing the gate valve to close and turn my hand so the flasks spout is pointing up.
At this point, I always, pour the measured powder into a separate container, set the flask aside and then pour the powder into the barrel or revolver cylinder.
This last step is very important because if there is a glowing ember in the barrel or cylinder, the new powder will ignite. Having a few grains of powder in the measure explode is entirely different than having the whole flask full of powder explode.
Over time, there have been many different methods of making the gate valve on powder flasks. Some simple like the ones on the Italian flasks and some, quite complex. The thing of it is though, none of the different designs are entirely safe so, if the newly measured powder does ignite, there is a chance of the flame getting past the valve into the new powder in the flask. That is why people who know, never pour from the flask into a freshly fired barrel or cylinder chamber.