What is a reason many of the world's largest cities are located near waterways?

When older cities were founded travelling by water was the fastest way to travel. Explorers would travel by water and see a place that was in a location that was suitable for trade or near a source of income (minerals or animal population) and build. Even if a location was on a seacoast they would also find a location where a river flowed into the ocean to provide fresh drinking water for them and their livestock.