I think for a **why** question to be a good one, it must reach some point of education that isn't just for knowledge of programming/computer trivia, but is useful information to know for programmers.

A good example is:

[Why Is Processing a Sorted Array Faster Than an Unsorted Array?](http://stackoverflow.com/questions/11227809/why-is-processing-a-sorted-array-faster-than-an-unsorted-array)

This question asks about a programming issue that is very relevant to most programming, and it has great answers that really go into the nitty-gritty details.

Another post that educates on a subtle point in programming:

[Why Does Python Code Run Faster In a Function?](http://stackoverflow.com/questions/11241523/why-does-python-code-run-faster-in-a-function)