Why would an author choose a third persn narrator?

Third person limited gives your readers access to a character's inner thoughts and emotions, much the same way that first-person narration does.

Third person limited gives your readers access to a character's inner thoughts and emotions, much the same way that first-person narration does. The difference is that there's a critical sliver of distance between the protagonist and narrator, which will change the way the main character is portrayed.

Explanation:

onto a large boat, and mark the water level that the boat sinks to. then, take the elephant out and put a bunch of marked weights onto the boat until it reaches the same water level. add up the masses of the weights to get the weight of the elephant.