What is the purpose of the rotating disk on the rim of a watch?

Question

I couldn't find anything about it on Google.

Answer 1

Imagine you have 30 minutes before your parking ticket or meter expires or you have agreed to meet someone in 30 minutes.  If you set the zero on your watch rim to the present minute hand position then the 30 on the rotating rim will show the time you need to be back at the car or to meet your appointment.  If you have an hour before your appointment is due then the zero, next time the minute hand reaches it, will be when you should be at your appointment.

Unfortunately rotating rims on some cheap watches are just for show and either don't move or are too easily bumped out of position. Otherwise they are good ways of reminding you how long to your next close appointment.  Also if you are in an unfamiliar place you can quickly work out how long it has taken you to reach where you are and and so approx how long it will take to get back to your starting point.

Answer 2

They were originally put there for SCUBA divers to mark their times under water.  They have to stay at certain depths for specific amounts of time to decompress or they get "the bends" (bubbles of nitrogen in the blood stream).  This makes it easy for them to set the next time interval for their present depth.