Why would I use es6 Set prototype -> 'entries'

Set and Map have slightly different APIs in terms of their stored values. While Sets contain values only, Maps contain their associated keys as well.

Because of this, should both of the structures have filter, map, forEach,... functions just like Array does, the action setting presumtion about each iterated value is likely to be slightly different, specifically:

Map: function(key, value) { }
Set: function(value) { }

Because when you already keep keys in a Map it makes sense to be able to inspect the value of the key during iteration as well.

Now if you encountered a situation where you would like to iterate over any of the two collections but cared only about the value, you would need two functions, one for each data collection.

map.forEach((key, value) => console.log(value));
set.forEach(value => console.log(value));

Even though you only care about the value and the logic of both actions is clearly the same, you cannot write a function to serve both collections. The entries method of both Set and Map fixes this issue by providing a common interface.

By creating the following function (or its alternative, I am not manily a JS developer so please excuse any inaccuracies):

function forEach(entriesIterator, action) {
    let current = entriesIterator.next();
    while (current.done === false) {
        action(current.value[1]);

        current = entriesIterator.next();
    }
}

you can feed either entries of Map and/or Set and it will still work.

forEach(map.entries(), value => console.log(value));
forEach(set.entries(), value => console.log(value));

TLDR: The entries method provides an abstraction to iterate over a data collection where you may potentially only care about the values.