Some subdials or mini-dials on watches are actually fake. They are there for only a decorative purpose. While others are quite useful. The working subdials are there to provide us with extra additional help when necessary. These mini-dials are auxiliaries located right on the face of a watch.
My collection of Invicta watches have them that I often use. These minidials have different functions depending on type of watch. Mine have several function abilities. For example; power reserve, to track seconds, minutes and hours. Even different moonphases, and other time zones.
If you DON'T know the name or have the watch manual, then search a similar watch on line for the particular information. Or a related watch video on YouTube. YouTube has always been a useful tool of mine. This way you can enjoy your watch to it's fullest, if you wish.