From Sal’s friend Katey S: two lessons for two days!
“The first is one that you taught Sally that I am constantly having to teach other people. It is the justification for the comma before “and” in a list of items. I knew the comma was important but couldn’t explain it well until Sally shared the example you used… of a will. If someone passed away and stated in her will she wanted to leave “$150,000 to Tom, Dick, and Harry” then Tom, Dick, and Harry would each get $50,000. But, if the will stated “$150,000 to Tom, Dick and Harry” (missing the imperative comma) then Tom would get $75,000 and Dick and Harry would each get $37,500. Big difference! Especially to Dick & Harry! I know it’s not exactly a moral mind-blowing life lesson, but boy people like to leave that comma out!
Note: This exact legal example came from a Business Writing class I attended in 1998. Different style guides have different opinions, but this held up in court for “Harry”, who actually got 50%, according to our teacher.
Jane's Lessons Learned 
Fascinating. I recall being taught that the comma you reference is optional, but this is really interesting to read the example!