Many folder-type (clamshell) phones have have secondary external displays on the outside, so that the user can see who is calling before answering in incoming call.
The display therefore shows Caller-ID information during an incoming call. Most phones display the time and/or status at other times.
Some phones have this 2nd display on the top instead of the front, so that it can be seen easily when the phone is worn on the hip.
Newer phones have 2nd displays that are color, which allows associated photos to be shown for incoming callers, a feature known as Picture ID or Photo Caller ID.
In many camera phones with full-color 2nd displays, the 2nd display can be used as a viewfinder, to frame self-portraits.