She's photogenic, without being beautiful. That's very useful for an actress.
Her large features register really well on film, they draw the eye and put over whatever it is she's trying to tell the audience. And the fact that she has regular features but isn't a great beauty makes her able to play a wide variety of roles; beauties can only play beauties, but Hathaway can play awkward regular girls or ordinary everywoman, or professionals who rely more on their brains than their looks, or somebody's girlfriend, or even, very rarely, a hottie. If you saw "Ocean's 8" (don't bother if you haven't), you'd see that she can play someone very glam and sexy if she wants to.
Maybe that is also similar to the appeal of Julia Roberts.. they both large features wraparound face smile very photogenic can be extremely beautiful or abit plain looking (well sort of).. as a result they are more appealing than actresses who can't be anything but stunningly beautiful? Idk
Yeah, Roberts also has those large features that register well on camera, perhaps the movie camera has an easy time reading their expression and nature gives them an edge in putting their feelings across through the camera. But other actors manage to do just fine with dainty or pale little facial features, so a Roberts/Hathaway sort of face is far from obligatory.
Roberts also has a good amount of that mysterious charisma called "star quality" or "screen presence", not as much as Marilyn Monroe or anything, but as much as anyone in the modern era. Now while I don't understand much about what makes a person photogenic, I don't understand ANYTHING about "star quality"! It's just there, or it isn't.