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 site you linked 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.
Das besondere Bandmaß mit 3 Funktionen:
- Kreise Die Teilung im Sichtfenster steht nicht auf dem Kopf (keine Ablesefehler).
Translation: "length measurement, inside measurement and circles. The markings in the window are not upside down (no reading error)"
The red line marks the inside measurement, taking into account the size of the tape's case.
It also marks the radius if you pull out a tab, and the offset due to the length of the tab is compensated by the location of the hole in the tape.
(But most of this was already in the earlier answers)