Nearly everything I own has at least one breast pocket: coats, shirts, fleeces and base layers. One of my waistcoat has three breast pockets (and eight others).
What goes into them:
Glasses.
Any ticket, paper, reference receipt etc. required for or received during this outing. Pen. Rulers, screwdrivers, paper handkerchiefs, the list goes on. Sometimes a spork.
Yep - sporks aren’t just for camping. I love ice cream. I am heavily bearded so always used to buy a tub with spoon - ie two single use disposable items. I now buy a cone which I shovel out with a folding steel spork.
Off topic 1. I use a steel spork at every meal including Christmas.
Off topic 2. I rarely wear any other trousers than the nine pocket Caribou.
Back on topic - Wearing my waistcoat and Caribous I have twenty pockets and use the breast pockets to the point of resembling a lumpy drag queen. The pockets that I never use are the inside ones which are often both/all breast pockets.