SICP pdf

I was just on hacker news and saw this http://web.mit.edu/alexmv/6.001/sicp.pdf
In the comments there was a different version http://deptinfo.unice.fr/~roy/sicp.pdf

I remember my first time reading SICP, I skimmed over the contents section and saw the bit on Register Machines. I thought to myself, what does computer programming have to do with cash registers?