Secondary camera is usually included in only those mobiles which are meant for video calling, because that is what the secondary camera is made for: video calling. Secondary camera is usually placed on the front side of the phone, mostly above the screen. While one performs a video chat, he or she would like to talk to the person face to face. This requires that the person has to show his 'face'. Since it is a lot easier and normal to look at the screen while talking rather than turning the phone backwards and facing the main camera, the secondary camera is used.
Most camera phones which feature video calling (such mobiles would also support 3G networks because of the high data transfer speeds required for the video calls) would include a front camera. The secondary camera of a phone is usually of low quality because they are made to focus only on the face of the person in front of the mobile screen. Mostly they can focus only on the near objects (up to 2 meters away) and do not come with high resolutions. Most front cameras on the mobile phones are of 640 X 480 pixels in resolution only. They should not be used for photography or shooting videos with the phone, when the primary camera is working.
The picture shows the position of the secondary camera in the Nokia 5800 Xpress Music phone. Most manufacturers place the secondary camera at the similar position.