I add quotes because it's a handy sort of quotes file (many people keep them) and because I like being able to reel off quotes or just have them handy in my memory for writing.

There's nothing fancy about them: the question is the quote, and the answer is all the sourcing and bibliographi information. I grade them based on whether I feel I could paraphrase them in a relevant context. ("Ah yes, good old Box's quote about how 'all models are wrong but some are useful'. Good to remember for statistical discussions. Mark that one a 4.")