What does a "nib cap adjuster" do?

[Charlie Schultz]

Here's the description in the Craft USA catalog, but I don't quite get it:
"for Fountain and Rollerball Pens
This adjuster is a must if one would like a precision fit between the cap assembly and the pen barrel. Just insert the adjuster into the cap assembly and the custom tip of the adjuster will slip into the nib cap for positive adjustment."

 

[Scott]

Hi Charlie,

On a snap-top kit, the part inside the cap that it snaps into needs to be adjusted for exact fit. This is most easily done with their adjusting tool. Of course it can also be done with a small, long screwdriver.

Scott.