Why Did Sumerians Use Sailboats?

Sumerians were great sailors. Discoveries of goods from far-away locations; goods such as obsidian from Anatolia, lapis lazuli from northeastern Afghanistan, beads from Bahrain suggest a remarkably wide-ranging trade business centered around the Persian Gulf.
 
The Epic of Gilgamesh, the original Noah story, refers to trade for goods such as wood that were scarce in Mesopotamia. In particular, cedar from Lebanon.
 
Sails and oars moved their ships up and down the Tigris and Euphrates Rivers, the Persian Gulf and the Indian Ocean.
 
Smart people, they invented the wheel and developed the earliest known system of writing.
 

Probably because the internal combustion engine had not been invented yet and sail power worked, and still works well, today. Sailboats require less manpower to move a boat than oars do, and it is a heck of a lot less labor to boot.
When tee is insufficient wind to inflate the sail, then you get out the oars and row.