Why do you need conventional signs on maps and plans?

Symbols dress up the use of maps very much, it can be said that without them the use of maps is either impossible or even meaningless. For example, to create a map with water temperatures around the world, you need to make rivers, seas of different colors depending on its temperature. Or, to cancel the largest cities, you need certain drawings indicating the approximate number of people. The larger the dot, the larger the population, and vice versa. Now electronic cards have replaced paper cards.