1) That's a hint that the garment was not made for diving in the first place.
Every undergarment I own has a breast pocket, and every one of them is diving specific (down to having venting on the left shouldeR). As other have mentioned, it's a spot to keep keys usually. I acutally get annoyed in the summer when I dive in light long johns and have nowhere to put my key (for my car I have an all metal key I can clip inside a pocket on my BCD, but with my wife's SUV I have to either leave the key with someone or hide it, or let if float around my drysuit in a baggy).
My undergarments also all have pockets on the hips too, which is nice for leaving a stocking cap in for SIs.