Can a former employer provide a useful recommendation for graduate admission to a science program? In its online application form for graduate admission, Stanford University requires three recommenders and allows for another three optional ones. Quoting the application form:


  You must provide three letters and evaluations from faculty or others
  qualified to evaluate your potential for graduate study. At least one
  letter and evaluation should be from a faculty member at the last
  school you attended as a fulltime student (unless you have been out of
  school for more than five years). Substitutions for faculty
  recommendations may include work associates or others who can comment
  on your academic potential for graduate work.


Reading this, I feel somewhat uncertain what type of recommendations would most help my application. I'm particularly confused by the phrase 


  Substitutions for faculty recommendations may include work associates...


I worked for Apple a few years back. A favorable letter from my then-supervisor might convey the impression that I'm technologically minded and enjoy ties to Silicon Valley. Considering Stanford's own proximity and close connection to the Valley, such a recommendation might be well-received?

On the other hand, I'm applying to the theoretical Physics program. My former supervisor at Apple clearly has no basis on which to judge my potential for graduate work in this field. Can anyone provide some pertinent insight into what purpose recommendations are supposed to fulfill? Would a letter solely commenting on my work ethic and team spirit help my case?
The important thing to keep in mind is that the goal isn't this specific letter, but composing the best possible application for, in this case, Stanford. So the primary question is: would a letter from your supervisor at Apple improve your set of recommendation letters? If you already have 6 letters from very qualified people that know your work well and can comment on your ability to perform at the Stanford program, a letter from your former boss might not add much. However if you only have 3 people at the moment who can really say something about you, adding your former boss couldn't really hurt. A few things in mind are:


How long, how long ago and how closely did you work with your former boss compared to the other people writing you letter of recommendation?  
Was there a (strict) selection process to get into Apple? If so, the selection people at Stanford might be interested why the selection people at Apple chose you out of all possible candidates.  
Did you do any research at Apple, or answer any research questions? Even when not doing theoretical physics, your attitude towards discovery and finding new knowledge might be relevant.