What is the 24-hour display for?
A 24-hour display doesn't show the day as two 12-hour halves, but as a complete 24-hour cycle.
This makes it easier for you to tell whether a displayed time means day or night.
A 24-hour display is especially common on GMT watches. There, an additional hand shows a second time zone on a 24-hour scale.
This is particularly handy when travelling or when staying in touch with people in other time zones.