Why add chapter quotes? Where do you get your chapter quotes from? Aren't chapter quotes hell to format?
Me, personally, I enjoy chapter quotes. Dorothy Dunnett, Seanan McGuire, and of course Frank Herbert are all awesome examples. If you've never read any of these authors, don't tell me because I will get very judgy.
"Facts are a commonly accepted interpretation. Truth is a commonly argued fiction." A Planet's Philsophy, Ankara Zaneth (From book 8...yes, I'm way ahead of myself.)
They're an insight into the world backdrop, a good laugh, or a context-setter, depending on what the author is doing with them and with their book. I put them in because, well, I'm a pure pantser. I don't outline. I generally have no idea what my characters are likely to do once I've dropped them into a scene. I find out when I write it down. As you can imagine, therefore, I usually end up writing my chapter quotes well after the fact. They're actually help me in the editing stage, because they act as a kind of focus mechanism for me when I'm editing a chapter. I can stare at the chapter quote for a bit when I get stuck, remember the awesome thing I was trying to do in that chapter, and return to hacking and slashing motivated and refocused. (Hah.) At least, that's how it sometimes works.
"Modesty is like arsenic: safe only in small doses." Sayings of the Wise, Olar Fantoml
As I kind of gave away in the last bit, I don't get my chapter quotes from anywhere. I make them all up. My father, who had very serious tastes in most of his reading, and considered sci-fi to be an extreme form of escapism, never actually read any of my books - but he would steal them from my mother when she was reading them, and he would read my chapter quotes. I still regret that I never really asked him why, because I think the answer would have been interesting.
"Avoidance requires continuous effort. Confrontation merely requires standing still." ~ Universal Truths, Jahira Suran
And yes, sometimes, depending on the platform, chapter quotes can indeed be hell to format. Kobo, for example, thinks my chapter quotes are a whole separate page unless I spend hours tickling it with an ostrich feather while immersing it in chocolate. (Kidding. I had to get much kinkier than that.)
"Training is not a substitute for experience; it is merely easier to survive." Training of a Cortiian, Nadhiri Longar (Yeah, Book 8 again...working on it.)
As to what my chapter quotes are supposed to achieve other than providing a focal point for my edits - I mostly leave that up to the reader. If they're something that you just skip on your way to the main events, no worries. If they make you grin, or start an interesting train of thought, then I'm happy. I frankly suspect most of mine actually come from Khyria's choices of reading matter. Most of them are downright cynical and sound like the kinds of things she'd remember.