Q:

what's the use of Edge Detection of image?

After getting edge image using canny, what's the use of edge image?
Is there any use case of edge image?
find object and Segment it from image? or get the sharp,area and perimeter of the object?

A:

As in the wikipedia,

Edge detection is the name for a set of mathematical methods which
  aim at identifying points in a digital image at which the image
  brightness changes sharply or, more formally, has discontinuities. The
  points at which image brightness changes sharply are typically
  organized into a set of curved line segments termed edges.

You can use this to find the interested area of an image by programmatically. For example, you have a lazer image of a indoor floor map and you want to detect the actual area a robot can visit, this will be useful. You can refer google more on this. It's just an example in real world usage. 

