Interesting how all above seems to have migrated toward a writing implement.
What about a pig pen? Is it not of value to the farmer? Maybe not as poetic, but who would Charles Schultz have named his dirtiest character after if not for the lowly pig pen?
Or how about the plight of the mail swan? He would be lost without his pen.
Or maybe most important of all would be the haphazard result of a squid who found himself pen-less.
Yes, there are more noble words offered by people with the pen, but not all pens are of the write kind that is not to make light of their need.
There are many different types of pens. As post 3 suggests, some medicines are contained in pre-measured packages that are called pens. These medications are placed in a plastic tube about the size of a large pen. Each medication is delivered through a needle which pops out of the tip during injection. Epipens are one example, but several diabetes medications also come in this form.
Post 4 talks about a fountain pen; a calligraphy pen is another similar example. Traditional calligraphy pens have a broad, thin metal tip that has a slit up the center. The pen is filled with ink or contains and ink cartridge. The ink is drawn from the pen, down the slit, and onto the paper. These types of pens are used for a specific style of writing. Each style of calligraphy requires its own specific shaped tip.
Edward Bulwer-Lytton contended that "the sword is mightier than the pen" in his 1839 play Richelieu: Or the Conspiracy. As an optimistic idealist, I hope that this philosophy is not true - I pray that the people of the world will eventually learn to talk or write to work out conflicts in preference to fighting.
I still pick up an old fountain pen and use it once in a while. It was always a challenge as a child to keep the ink from clotting or smearing, but I still think it provides the most beautiful form of penmanship ever.
If you are looking for unique uses of pens, some medicines are described as pens. I am thinking of the epipen, which is used to administer medicine to people who are in anaphylactic shock from an allergy attack. Here is a link describing how it works:
What is the use of a pen?
Well, you can toss it up and catch it. You can tap it against things. You can play with the click-top. You can use it to open soda cans, or throw it to your cat, or spear a pickle out of the jar, or wedge it under a window to keep it open, or poke holes in a box, or hang it from a string for a mobile, or set it on the floor to show where your side and my side meet.
Oh, and you can write with it.