The purpose of imagery and description in writing is to help the reader get a better idea of what is being described. It is important because if we didn't use it, we would have no idea what things looked like, smelled like, or anything like that. We would have no idea what is being discussed.
Here's an example:
"The leaves fell gracefully, crinkling atop eachother while the glistening sunlight made them sparkle."
That sentence would be a lot more boring and dull if we didn't use imagery.
