The feature is a big deal for Facebook since photos are a core driver of return visits and time on site. Most people take way more photos with their phones than they upload to Facebook and the social network wants to close the gap. Instead of having to manually choose photos and wait for them to upload, Photo Sync makes this all very low friction, which could drastically boost the number of photos people publish.
You could upload all your photos to Facebook and restrict their visibility to only you. Thus, when you’d want to put a photo on Facebook, it’ll already be there. This is big for them; perhaps they should expect that Apple will block this.