It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that when the Noble Prophet (peace and blessings be upon him) would go out on the day of Eid, a spear would be fixed in front of him by his order, and the Noble Prophet (peace and blessings be upon him) would use it as a sutrah and lead the prayer, and the people would be behind him. The Noble Prophet (peace and blessings be upon him) would do this while traveling as well. From here, the rulers have adopted this practice.