Why such photos are always cropped square

All these great masters photos has something in common and their subjects are quite identical to each other though there are also varieties in between them. They are all cropped square. Is there a special visual impact that it can bring out when cropped square instead of rectangle?

http://www.flickr.com/photos/sleachim/

http://www.flickr.com/photos/tjintjelaar/

http://www.josefhoflehner.com/portfolios.html

http://www.michaelkenna.net/

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David