Symbols substitute for images in the thought process.
The arbitrary concept/symbol association, has one major advantage for communication--boundless POWER!
There is no limit on the concepts that can be represented by symbols. Hence, there is nothing that cannot be communicated via language!
One final difference between signs and symbols is that the signs relate to things, but symbols are substitutes for them in our thought processes.
Dark clouds for example mean that rain may be eminent but we don‰t substitute the image of dark clouds in our mind for rain.
The word "rain," however, is a substitute in our thought processes for the real thing or image. In fact most of us as adults chose to think verbally (in words) rather than in images.
Other symbolic forms (which probably preceded language in the course of human development) include rituals, music, dancing and art.