The overfeed extends the width and range of lines you can draw. It rests above the nib, stores more ink between the nib and overfeed, and controls the degree of flex, which lets you draw wider lines for a longer time. Read more about our overfeeds.
You can replace the overfeed with a spacer when you don't want to use the overfeed. Slip overfeeds and spacers in and out.
About the reservoir
You can double the volume of ink or paint stored in the pen by removing the reservoir and filling the body. The reservoir provides a double wall of protection, which might be what you want, but you can carry more volume without the reservoir.