The property list provides an easy way to create a database of relatively permanent facts about objects, e.g., the parts of speech of a word. It is convenient because additional kinds of facts can be added without interfering with existing facts, so long as the property names are different.
(setf (get 'car 'part-of-speech) 'noun)Note that the symbol car can play several roles: it can hold facts about the English word ``car'' on its property list; it can be used as a variable name; and it is the name of a system function. These uses of car are entirely separate and do not conflict.
The property list provides an easy and safe way to create networks of relationships among objects (sometimes called semantic networks) while avoiding potential problems due to circular structures and multiple references to the same entities. These problems are avoided because a symbol is always guaranteed to be a single, unique structure in memory.