Lambdas in FunctionalInterfaces in Java

I am trying to make use of lambdas in Java but can't understand how it works at all. I created @FunctionalInterface like this:

@FunctionalInterface
public interface MyFunctionalInterface {
    String getString(String s);
}

now in my code I use the lambda as here:

MyFunctionalInterface function = (f) -> {
    Date d = new Date();
    return d.toString() + " " + person.name + " used fnc str";
};

Next, I want to make use of my function passing it into the constructor of another class and use it like this:

public SampleClass(MyFunctionalInterface function) {
    String tmp = "The person info: %s";
    this.result = String.format(tmp, function.getString(String.valueOf(function)));
}

Why I need to use it the valueOf() here? I thought that thanks for this I could use just function.getString()?

Output: Tue Sep 19 11:04:48 CEST 2017 John used fnc str