first, protect the pen tip. Putting the brush into it won't damage the pen tip as if it were packed in a pencil box.
second, ventilation and mildew prevention. The pen curtain is ventilated, which can make the brush quickly dry and cool.
third, use: I need to take my writing brush out in an emergency, but I just used it and didn't dry it. You can roll it with a pen curtain and take it out. Will not be moldy. There are more pens for drawing than for calligraphy, so it is more convenient to draw when you stand beside them.