Why did the aztecs build a shrine to tlaloc, the rain god on the top of the great temple

The Great Temple (templo major) in Mexico had two shrines on top of it, and one of them was dedicated to Tlaloc.
Tlaloc, the god of Rain, is one of, and possibly the most important God in Pre-Columbian Mexico, which by itself was a good reason to built a temple for him on top of the Templo Major, an important temple complex in the capital. The purpose of the temple was to plead for rain and agricultural fertility, a very important aspect of live for the Aztecs. This was believed to be ensured by human sacrifices in the temple. The Aztecs believed that this would lead to a great harvest, and prosperity as a result.