Why is land next to a waterway or a large lake a good place to start a community?

Because they have water.That is the main reason they do it,like in Arizona most of the towns and cities had to be built near a river.Arizona has few rivers so they have cs and redirected waterways to bring them water or else they would have to make trips to the closest river,bring it back to a filtering place,then bring it to the community to drink.But with it so close you wont have to wait long if you need water.