i bet ten-bucks the answer is "water". no one can live without water.
Trade,building materiel shipment....and most importantly,it watered their crops and provided food.
The used Nile water to brew delicious and refreshing beer.
Oh dear, let me think. Maybe - just a guess, mind - because it was their only source of water?
It was and still is the only source of good quantities of water for irrigation of agriculture. In ancient times boats on the river were also the only reasonable form of transport from one end of a very long country to the other.
It distributes fertile soil along its delta.
It was the source of life.
It provided water and nutrients to grow crops and was a means of transportation.