Why do authors vary sentence structure in their writing

Read the excerpt from part 2 of zeitoun. zeitoun woke with the sun and crawled out of his tent. the day was bright, and as far as he could see in any direction the city was underwater. . he could only think of judgment day, of noah and forty days of rain. and yet it was so quiet, so still. nothing moved. he sat on the roof and scanned the horizon, looking for any person, any animal or machine moving. nothing. as he did his morning prayers, a helicopter broke the silence, shooting across the treetops and heading downtown. why does the author include the details of zeitoun’s morning? to suggest the hopelessness of zeitoun’s situation to illustrate the solitary existence endured by survivors to establish zeitoun’s dedication to the rituals of his faith to reveal the military’s attempts at assistance