Why do authors (like Shakespeare) use allusion?

Question: Why do authors (like Shakespeare) use allusion!?

I think I know, but asking certainly can't hurt
Allusion as in mythological, Biblical, historical, obviouslyWww@QuestionHome@Com

Best Answer - Chosen by Asker:

To add to the above, allusions often take the role of a forshadowing of some sort - for example, if someone now named a fictional character Romeo we may think immediately of "Romeo and Juliet"; then, we may think, "does this character have a similar tragic end!? or does he escape it instead!?"

Many allusions used by authors, not just Shakespeare (though he certainly did use many, despite my lack of ability to point them out for you, sorry), to link new characters or story lines with ones people are aware of, and to get feelings of expectancy!.
Sometimes the relation is intended to be criticizing what is being alluded to, though most of the time this role of the allusion will not be properly found by the reader until the very end of the story, in order to encompass all new details!.Www@QuestionHome@Com

It relates one idea to another!. Not in the same way that a simile or metaphor does so, but more through referencing back to prior knowledge of other subjects!.

Any idea, in isolation, seems irrelevant!.
But when allusion is used the reader gets a sense of relevance, capturing their attention!.Www@QuestionHome@Com