My personal favorite part is that you don't have to do this mental math of "the frame is this much, then pick lenses you want depending on your budget, etc." The price of the frame is the only thing you pay, all the fancy lenses and such are free, whether you need them or not (in case of sunglasses). With frames being noticeably cheaper than other brands and lenses being free (usually the bulk of the cost of prescription glasses), they are welcome to put their hand in my wallet.
The only reason I don't shop there much anymore is because of LASIK. Back then though, I had about 4 different frames from Warby Parker, and those were my favorites, despite previously wearing the usual "known" eyewear brands (and that's even without factoring in the cost).
P.S. There are some extra charges for lenses in rare scenarios, like if you have a very very strong prescription, they charge you a bit extra if you want to make the lenses thinner. Still not comparable to traditional glass brands though in terms of how much it costs.
P.P.S. There are other affordable online eyewear places, but I have been appalled with the quality and look of those. A couple of friends of mine tried Zenni Optical before, and those frames felt flimsy and they didn't look good at all imo. Warby Parker seems to have found a really solid niche between "cheap and flimsy" and "overpriced brand name eyewear", which is "the quality and looks of premium eyewear, but with a much more reasonable pricing".