Object detection does two things, it localizes objects in a picture and then classifies the object. So one both gets the coordinates of the object and the classification of the object. Or put simply, it detects objects within a picture and draws a box around them.
Object detection is well suited in situations where one needs to detect multiple distinct objects in the same picture. Then depending on the application one can use the information for tracking, localization, counting etc.
Like most things in AI, it can be applied in a vast array of different situations. Here are some examples of object detection.
Tracking objects in a video
People detection in Security
Counting the number of items in a picture
Finding out if a specific type of object is in a picture, for example, detect if q detect is present