More of the subject can be fit within a frame when an odd angle is used. If it was totally vertical, then only the head and chest would show, and then the legs would be cut off. Or if the legs were still in the image, then it would be small to show everything from the head to legs and there would be empty space to the left and right.
Viewing the subject on a diagonal would show the most of the subject because mathematically the hypotenuse of a right triangle is longer than the horizontal and vertical lengths.

See dutch_angle and/or forum #8433

sakami-chan said:

More of the subject can be fit within a frame when an odd angle is used. If it was totally vertical, then only the head and chest would show, and then the legs would be cut off. Or if the legs were still in the image, then it would be small to show everything from the head to legs and there would be empty space to the left and right.
Viewing the subject on a diagonal would show the most of the subject because mathematically the hypotenuse of a right triangle is longer than the horizontal and vertical lengths.

just say a^2+b^2=c^2