In official module's source code I found some call to class constructor:

class hr_holidays_summary_employee(osv.osv_memory):
    _name = 'hr.holidays.summary.employee'
    _description = 'HR Leaves Summary Report By Employee'
    _columns = {
        'date_from': fields.date('From', required=True),
        'emp': fields.many2many('hr.employee', 'summary_emp_rel', 'sum_id', 'emp_id', 'Employee(s)'),
        'holiday_type': fields.selection([('Approved','Approved'),('Confirmed','Confirmed'),('both','Both Approved and Confirmed')], 'Select Leave Type', required=True)
    }

    _defaults = {
         'date_from': lambda *a: time.strftime('%Y-%m-01'),
         'holiday_type': 'Approved',
    }

    def print_report(self, cr, uid, ids, context=None):
        data = self.read(cr, uid, ids, [], context=context)[0]
        data['emp'] = context['active_ids']
        datas = {
             'ids': [],
             'model': 'hr.employee',
             'form': data
            }
        return {
            'type': 'ir.actions.report.xml',
            'report_name': 'holidays.summary',
            'datas': datas,
            }

hr_holidays_summary_employee()

Why use hr_holidays_summary_employee() here?