The Sumerian King List is a Sumerian-language cuneiform text that chronologically lists the rulers of the major city-states, beginning with the antedeluvian descent of kingship from the heavens, up until its composition circa 1900 BC during the Isin Dynasty. Its final version was used by the Isin kings to legitimize their claims to supreme power by depicting them as the culmination of an ancient lineage that spanned many cities (even if they were not hegemonic over the whole area covered by the King List).
The Sumerian King List is largely legendary, but towards the end its accuracy improves and can be confirmed by other sources. While early periods include impossibly long reigns (tens of thousands of years) held by deities, by the Early Dynastic periods the information becomes more accurate. This provides invaluable name, reign and sequence information, particularly when the kingship lands at Isin and the informations becomes quite factual.
Grand totals of dynasty lengths further add to the Sumerian King List's value to scholars. Also, the text informs modern scholars about ideas regarding kingship: that it descended from the heavens, and could only be held by one ruler at a time. While ideology said there was only one divinely legitimate ruler at a time, the reality was that kings were often contemporaneous and in conflict. The text compromises and depicts a rotating kingship that transfers from city to city (often landing at Ur, Uruk and Kish).
Chronologically, the text addresses the period from the moment kingship first appeared, before the flood, to the dynasty of Isin (ca. 1900). ... In its final version, it was used by the king of the Isin dynasty to legitimize their claim to supreme power in Babylonia, even though they did not politically control the entire area covered by the King List. Mieroop, p 43
In the segment that covers the Early Dynastic period, the city-states mentioned are primarily located in Babylonia, giving special prominence to the cities Ur, Uruk, and Kish. Also included are three non-Babylonian cities, Awan in the east, Hamazi in the north, and Mari in the west. From other evidence we know that some of the kings listed consecutively ruled concurrently. The text enumerates them sequentially because the main ideological elements expressed in this text are that there is only one divinely legitimized ruler at a time, and that hegemonic kingship circulated among a restricted number of cities. Incorporated in it were dynastic lists of kings from different cities and he number of years they ruled. Mieroop, p 43
The accuracy of the later parts can be checked against information from dated economic documents. The earlier parts of the Sumerian King List are legendary, however, assigning impossible long reigns of 3600 years, for instance, to mythological figures such as Dumuzi, who was known as the husband of the goddess Inanna and was probably purely fictional. Mieroop, p 43