The seraphim serve the function of announcing the Lord. This is a formal task, not a necessary on. That is, these “burning ones” are serving as the court proclaimers of the sovereign. The announcement of his glory is ritual, because Isaiah has already understood who he was. The phrase “the whole earth is full of his glory” emphasizes the glory of the Lord, and underscores the seraphim’s coverings as protection from that sovereign glory.