Final answer:
The primary purpose of the letter was to inform other Texans that reinforcements were needed at the Alamo.
Step-by-step explanation:
The primary purpose of the letter mentioned above was to inform other Texans that reinforcements were needed at the Alamo. After holding the mission under siege, Santa Anna sounded the degüello, declaring a battle to the death. The defenders of the Alamo, led by W.B. Travis, declared there would be no surrender or retreat, and the letter was sent out to request reinforcements from other Texans to help defend the Alamo.