What does the symbolic level of an allegory accomplish?

Not knowing whether this question is directed at a specific piece of writing or not, I will answer the question generally.
The symbolic level of an allegory accomplishes many things, such as the subconscious and conscious rates in which the true meaning is revealed or figured out by the reader, the understanding of the meaning, and the chance to analyze the symbols directly and indirectly.
For example, the level of symbolism can make the piece more thoroughly understood or more difficult to see the big pictures.  If a piece is filled completely with symbols (whether they be situations, people, animals, places, or objects), the hidden meaning will be easier to understand, as it would not be a complete story without the underlying meaning.  If the story is full of symbols AND things that do not directly or indirectly relate or point to the message, there will be more confusion for the reader and it may take them longer to understand that the story is not just a story, but a facade for this hidden meaning.  Feel free to ask questions and good luck! :)