
Extract the answer to the question from the following context.
Question: What purpose did the treaties serve?
Context: Relations between the Rus' and Byzantines became more complex after Oleg took control over Kiev, reflecting commercial, cultural, and military concerns. The wealth and income of the Rus' depended heavily upon trade with Byzantium. Constantine Porphyrogenitus described the annual course of the princes of Kiev, collecting tribute from client tribes, assembling the product into a flotilla of hundreds of boats, conducting them down the Dnieper to the Black Sea, and sailing to the estuary of the Dniester, the Danube delta, and on to Constantinople. On their return trip they would carry silk fabrics, spices, wine, and fruit. The importance of this trade relationship led to military action when disputes arose. The Primary Chronicle reports that the Rus' attacked Constantinople again in 907, probably to secure trade access. The Chronicle glorifies the military prowess and shrewdness of Oleg, an account imbued with legendary detail. Byzantine sources do not mention the attack, but a pair of treaties in 907 and 911 set forth a trade agreement with the Rus', the terms suggesting pressure on the Byzantines, who granted the Rus' quarters and supplies for their merchants and tax-free trading privileges in Constantinople.

set forth a trade agreement with the Rus', the terms suggesting pressure on the Byzantines
