Good explanation of why it's optional in this case, although i'm not convinced that reason is the only reasonable antecedent of why.
For example, the explanation why is a common usage,.
Why should the first person pronoun 'i' always be capitalized?
I realize that at one time a lot of nouns in english were capitalized, but i can't understand the pattern of those.
I have flunked the exam, that is why i am attending coaching classes.
I have flunked the exam, which is why i am attending coaching classes.
Is there any change in the meaning of sentence.
I don't know why, but it seems to me bob would sound a bit strange if he said, why is it that you have to get going? eliminating 'that' before 'bob' would seem to be more in context.