AngularJS $scope vs this

I’m beginner using AngularJS, and I have a question to ask.
I call a rest api to get comments by blog id, like following:

_getAllCommentsByBlogId(1, _convertComments);

function _getAllCommentsByBlogId(blogId, callback) {
    $http({
        method : "GET",
        url : "/api/v1/blogs/" + blogId + "/comments"
    }).then(function(res) {
        // success
        callback(res.data);

    }, function(res) {
        // error
        console.log("Error: " + res.status + " : " + res.data);
    });
}

The response data is converted for view task, like following:

function _convertComments(response) {
    $scope.comments = convert(response);
}

function convert(array) {
    var map = {};
    for (var i = 0; i < array.length; i++) {
        var obj = array[i];
        obj.children = [];
        map[obj.id] = obj;
        var parent = obj.parentId;

        if (!map[parent]) {
            map[parent] = {
                children : []
            };
        }
        map[parent].children.push(obj);
    }

    return map['0'].children;
}

After converting, i can view it as follows:

<div id="commentsApp"
          class="response-area"
          ng-app="commentsApp"
          ng-controller="CommentsController as c"
        >
          <h2>3 RESPONSES</h2>

          <div>
            <script id="comments.template" type="text/ng-template">
              <div class="comment">
                {{comment.username}}
                <div ng-if="comment.showReply">
                    <div class="form-group">
                        <textarea autofocus class="form-control" row="3" ng-focus="c.showReply" ng-model="c.newComment.description"></textarea>
                    </div>
                    <div class="form-group">
                        <button class="btn btn-danger" ng-click="c.reply(comment)">reply</button>
                        <button class="btn btn-danger" ng-click="comment.showReply=false">cancel</button>
                    </div>
                </div>
                <ng-include src="'comments.template'" ng-repeat="comment in comment.children"></ng-include>
              </div>
            </script>
            <ng-include
              src="'comments.template'"
              ng-repeat="comment in comments"
            ></ng-include>
          </div>
        </div>

I try changing following part in view

<ng-include
              src="'comments.template'"
              ng-repeat="comment in comments"
            ></ng-include>

into

<ng-include
              src="'comments.template'"
              ng-repeat="comment in c.comments"
            ></ng-include>

and following part in js

    function _convertComments(response) {
    $scope.comments = convert(response);
}

into

    function _convertComments(response) {
    this.comments = convert(response);


}

then the view doesn’t show anything. I already worked in this way and everything seems to work as expected.
Can you explain that?

I also read 'this' vs $scope in AngularJS controllers but my english is not good enough to understand it.
Thanks

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