Allusion provide a deeper understanding to a piece of literature by referencing something familiar
Indirect of passing reference
Allusions can give a deeper meaning to a story by referring to another piece of work that most are familiar with.
If a character within a story uses an allusion (refers to another piece of work), it can give deeper insight on what kind of person they are.
For example, if a character named Jimmy enjoys reading books like Romeo and Juliet, that shows the reader what they value and how they are.
On the other hand, if Jimmy enjoys reading Spider Man comic books, that gives you a whole new out look on his personality.
The effect of allusions pretty much just provide deeper understandings and more relatable situations.