Why choose a rectangular surface for flux calculation near a wire?

[darksyesider]

Homework Statement

http://web.mit.edu/viz/EM/visualizations/coursenotes/modules/guide10.pdf

pg: 10-17

Homework Equations

The Attempt at a Solution

I do not understand conceptually, why we chose the rectangular surface to take the flux over. Can anyone explain this please? i am having great trouble with it.

thank you.

 

[issacnewton]

It just makes it easy mathematically to demonstrate the calculation. Any other weird shape will need numerical techniques to do the integral.