I'll give my experience in lieu of Ben's - I have dual citizenship, Dutch and American (husband is Russian). Every country has very specific passport photo requirements. I have tried the passport selfie applications and have had no success - it's hard to get the right background, lighting, and head size. In renewing Dutch and US passports I've had passport photos rejected even from professional photo places. When I renewed an ID last year in Holland, the photography place was was next to the office that renews IDs, so it does a ton of them. Even the photographer there even said sometimes they reject the photos, even though he believes they conform to the passport standards.
Secondarily to that is that the consular interview is incredibly stressful. People are on edge because there is so much at stake. People get denied visas because they can't produce the documents the office deems they need, and from what I've read here on the forum, it is not inconceivable they may ask for something you don't expect or have on you. So it is important to reduce any potential issue and get the things right that you CAN get right, such as passport photos. Last thing I would want is to be in a foreign country paying for expensive lodgings and being on a travel deadline and then have your application denied because of your passport photos and then somehow hope you can get a new appointment. To me it's a simple thing to avoid, just go to a reputable photo place and verify they do US passport photos and have experience.