Symbols help to give more meaning to maps.

Customise - the symbol colour can be changed;
Combine - the symbols can be edited and combined to create new ones;
Experiment - the symbols can be used in many ways, including desktop GIS, web cartography and desktop publishing software;

Maki - is an established icon set made for map designers;

The Noun Project - is an impresive library of icons, useful in map design;