Why did sumerians need to trade

As any other ancient civilization, the Sumerians were traders as well. And they had a few good reasons to do this. 

Because they traded, their trading partners were less likely to attack them, because it would hurt their economy as well. Furthermore, they didn't produce everything they needed and traded those goods with goods which they produced too much of. 

And lastly, through trade they could also get exotic goods which they would have never gotten otherwise.