Mosiah 2:7

for the multitude being so great, that king Benjamin could not teach them all within the walls of the temple; therefore, he caused a tower to be erected, that thereby his people might hear the words which he should speak unto them.

The crowd was so big that King Benjamin could not teach them all inside the temple walls. So he had a tower built so his people could hear the words he would speak to them.

The crowd was so massive that Benjamin couldn’t fit everyone inside the temple walls, so he had a tower built. From there, everyone could hear him speak.