Sometimes UML diagrams get too big. You want to show all the important relationships, but everything is starting to look like a big mess.
In UML it is possible to represent multiplicity in a class. Say I have a Patient, and this patient has many addresses, like thus :
This saves on space in the diagram assuming you really want to show this relationship (and not just leave it out altogether). Obviously the internals of the Address class do no exist here, but depending on what you are trying to communicate in the diagram, these details may not be important anyway.