Can someone please explain me the significance of the window in this measuring tape? I am unable to understand its application.
That measurement window is used when you're using the tape measure to draw a circle.
This diagram (from the dire you liked to) shows how it's done:
There appears to be a piece which you pull out from the back of the body of the tape measure which you would hook over a nail or something at the center of the circle, and there is a hole in the tape at the 2.5cm mark where you insert your marking or scribing tool.
The reading in the window on top then tells you the distance from the nail to the mark - which is the radius of your circle.