Well, a reason is because it provides an accessible outlet for transportation, water, and waste disposal. So having a source of water means they can have a way to make transportation, like boats. Also, even a small reason why urban centers are developed along waterways is that there is accesible water.
This is what I found. Raindrops form when microscopic water droplets bump into each other in clouds. the more turbulent the clouds the bigger the raindrops get
