If they lacked a written language they wouldn't have the ability to document the trade/deal etc.
Explanation: Incase the other person decides to do something deceitful like break one of the rules of the trade conducted,Ancient people would need evidence that they signed and agreed to the deals' rules. Since Most couldn't read or write in ancient times, They created the cylinder seal to give evidence of the deal taking place, either that or they would have scribes write out most of it for the peope conducting the trade etc.
