When you take a new photo, it synchronises with all your devices, and therefore you see it on your Mac, iPhone, etc. However, if you get a new Mac (I got one be

When you take a new photo, it synchronises with all your devices, and therefore you see it on your Mac, iPhone, etc. However, if you get a new Mac (I got one because my library was under capacity), Photos will not start synchronising your 10-year-old photos until you process them through the phone. I hope I've made it clear now.