实体类的用途和好处是什么?

我经常遇到如下代码(参考 Slim tutorial 在 github )

ticketmapper.php文件

class TicketMapper extends Mapper
{
    public function getTickets() {
        $sql = "SELECT t.id, t.title, t.description, c.component
            from tickets t
            join components c on (c.id = t.component_id)";
        $stmt = $this->db->query($sql);
        $results = [];
        while($row = $stmt->fetch()) {
            $results[] = new TicketEntity($row);
        }
        return $results;
    }
    /**
     * Get one ticket by its ID
     *
     * @param int $ticket_id The ID of the ticket
     * @return TicketEntity  The ticket
     */
    public function getTicketById($ticket_id) {
        $sql = "SELECT t.id, t.title, t.description, c.component
            from tickets t
            join components c on (c.id = t.component_id)
            where t.id = :ticket_id";
        $stmt = $this->db->prepare($sql);
        $result = $stmt->execute(["ticket_id" => $ticket_id]);
        if($result) {
            return new TicketEntity($stmt->fetch());
        }
    }
    public function save(TicketEntity $ticket) {
        $sql = "insert into tickets
            (title, description, component_id) values
            (:title, :description, 
            (select id from components where component = :component))";
        $stmt = $this->db->prepare($sql);
        $result = $stmt->execute([
            "title" => $ticket->getTitle(),
            "description" => $ticket->getDescription(),
            "component" => $ticket->getComponent(),
        ]);
        if(!$result) {
            throw new Exception("could not save record");
        }
    }
}

票务.php

class TicketEntity
{
    protected $id;
    protected $title;
    protected $description;
    protected $component;
    /**
     * Accept an array of data matching properties of this class
     * and create the class
     *
     * @param array $data The data to use to create
     */
    public function __construct(array $data) {
        // no id if we're creating
        if(isset($data['id'])) {
            $this->id = $data['id'];
        }
        $this->title = $data['title'];
        $this->description = $data['description'];
        $this->component = $data['component'];
    }
    public function getId() {
        return $this->id;
    }
    public function getTitle() {
        return $this->title;
    }
    public function getDescription() {
        return $this->description;
    }
    public function getShortDescription() {
        return substr($this->description, 0, 20);
    }
    public function getComponent() {
        return $this->component;
    }
}

我当前的实践不使用实体类,而我的映射器方法只返回如下所示的stdclass:

class TicketMapper extends Mapper
{
    public function getTickets() {
        $sql = "...";
        $stmt = $this->db->query($sql);
        return $stmt->fetchAll(PDO::FETCH_OBJ);
    }
    public function getTicketById($ticket_id) {
        $sql = "...";
        $stmt = $this->db->prepare($sql);
        $result = $stmt->execute(["ticket_id" => $ticket_id]);
        return $stmt->fetch();  //Assuming my PDO is configured to return an object only
    }
    public function save($ticket) {/* no change */}
}

为什么数据库结果经常包装在某个实体类中?有什么标准可以决定是否这样做吗?