Crown at 10 o'clock

Does anyone know what the crown at 10 o'clock is used for?

Great Looking Watch

My wife gave this to for Valentines Day. It looks amazing. Took a minute to figure out the bezel ring crown, but that is part of the adventure. It is heavy enough to tell you that it is there, but not too heavy. Color contrasts are great, as I am color blind it is a big deal to me.