Difference between React Ref property by string and function

I'm new to React web development, and I'm studying about Component ref. I found this note:

You can allocate a function as a ref property of JSX. This function runs only once when the element is mounted. In this function, you can put a DOM node into instance's property.

<input ref={(input) => {this.emailInput = input}}
  className="form-control"
  type="text"
  placeholder="cad.co" />

In current official document, it is recommended to use function for ref, and it is mentioned that string ref will likely be deprecated in future version(https://reactjs.org/docs/refs-and-the-dom.html#legacy-api-string-refs).

I roughly understand that refs and ref are used as reference variable, and also that for ref property string literal can be used.

But I don't get the point about how to use ref property with function argument. How can it be the same or similar to the work of using string property for ref?