The deal is that they show the radiologist where the nipple is, leaving them less likely to misidentify it as a lump or suspicious spot when the nipple ends up squished or otherwise positioned under other breast tissue on the film. (Well, digital file, but I still call it film.

There's no modesty value. Indeed, i've never before had a mammography tech do anything that led to their fingers apply direct pressure to my nipples and it's hard to say the actually cover anything. The ones they put on me were mostly transparent, with some little black lines making a target.

There appear to be a number of different styles of nipple stickers on the market. The mind croggles.