对象构造函数除了分配其依赖项之外还有什么问题？

问：

我看到了一些文字，上面写着“在构造函数中什么都不做，只分配属性......”，然后是重构代码之前和之后的下面。说实话，我一直在遵循这种做法，但想知道和理解一些更好的例子，如果不遵守这个准则，事情可能会出错。

重构前：

final class MySQLTableGateway
{
    private Connection connection;
    public function __construct(
        ConnectionConfiguration connectionConfiguration,
        string tableName
    ) {
        this.tableName = tableName;
        this.connect(connectionConfiguration);
    }
    private function connect(
        ConnectionConfiguration connectionConfiguration
    ): void {
        this.connection = new Connection(
            // ...
        );
    }
    public function insert(array data): void
    {
        this.connection.insert(this.tableName, data);
    }
}

重构版本

final class MySQLTableGateway
{
    private ConnectionConfiguration connectionConfiguration;  
    public function __construct(
        ConnectionConfiguration connectionConfiguration,
        string tableName
    ) {
        this.connectionConfiguration = connectionConfiguration;
        this.tableName = tableName;
    }
    private function connect(
        ConnectionConfiguration connectionConfiguration
    ): void {
        if (this.connection instanceof Connection) {
            return;                                             
        }
        this.connection = new Connection(
            // ...
        );
    }
    public function insert(array data): void
    {
        this.connect();                                       
        this.connection.insert(this.tableName, data);
    }
}