A copy of Mao's little red book, it's easily accessible and can be pulled out to smite liberals.

Pack of smokes 🤷‍♂️

It's where you keep your pens, like a true nerd.

pens, pencils, slide rules, notebooks. I think it's a holdover from when we used to have analog math. You can store stuff in it and access it while you're sitting at a desk or whatever without standing up. This is all conjecture I don't actually know.

Related: why do baby clothes have pockets? What do they need to carry? Pretty rocks, sticks and sand of course, but whats the non-joking rationale behind giving babies pockets?

For cool truck drivers, Cigarette. For nerds, a calculator. #18wheelsmovetheworld

It serves an aesthetic function by way of breaking up the uniform look of the front of your shirt, giving it some nice asymmetri.
The pocket used to be bigger so you could have a little note book or a pack of ciggies or something in it, but I've noticed shirt makers often make it unuseably small now.

https://youtu.be/E0ZpQADBsEo

So you can drop your phone in the toilet when you bend 90 degrees to flush

I am a mailman. I keep so much shit in that pocket. Pens, markers, little thumb thimble that helps grip letters, employee id, sunglasses, cards to leave if someone isn’t home, airpod case… it’s actually a surprisingly spacious pocket lol.

Also I don’t smoke but for 90% of people who smoke who wear workwear that is the official pack of smokes pocket. In your pants pockets they get smooshed.

vape pen

that's where I keep my beanis

I work in the woods. I keep pens, a compass, a roll of flagging, and ear plugs in those pockets.

I put my sunglasses in it (lenses in, one of the ear pieces out). It feels nicer than on top of my head or on my collar.

Pens, loose screws, cigarettes.

i put my earphones i nthere when im not using them

Pens, for me, usually

I just checked mine, it's full of rubber bands.

The only purpose I've found is when I play card games with my friends and they ask me (for e.g.) hey ppintandclique where are your hand cards and I say huh? Handcards? Sorry, I only have pocket cards and then I take my cards out of the breast pocket

Gets a laugh every time

I put my wallet there sometimes.

Pencil

I have one in my scrub tops! I keep my little notebook and pens and stop watch in it

I put my nicotine gum in shirt pockets.

Candy

I put my phone in it so i can listen to tunes and podcasts without headphones

It's a fashion design element from when the pockets would be functional.

You don't really see them on dress shirts in either the UK or the USA, can't recall which.

Sunglasses or qtips, spare joints or cigarettes (or lighters or papers), a lil cash for cover. Keys even. A receipt

I actually hate t shirts without one, it's incredibly handy. Pocket t shirts ftw

I think it's a hangover from the pocket watch days.

That or its for pens

Put your phone in there, and then drop it in the toilet when you bend down to raise/lower the seat.