For hours, al-Fadl had trudged through sand and shouted fruitlessly
for help. Now, as the severely dehydrated cleric collapsed into
the suffocating sand and felt his heart give out, he asked himself the
same question he had been asking for hours.
a descriptive phrase "the severely dehydrated cleric" is used instead of pronoun "he". I was wondering what the purpose of this kind of writing technique is. An obvious effect is to introduce the description ("severely dehydrated"). Are there any others?
You are right in your assumption that the statement introduces a description of the subject. But more than that, it introduces a reason for the second sentence. If al-Fadl was trudging through the sand with two 5-gallon water cans (over 80 pounds), he could collapse due to exhaustion or heat stroke instead of dehydration. Although the context leaves little to doubt in the cause of the second sentence, the statement clears all doubt about the reason for his collapse.