The purpose of the convolutional layers of a CNN is to perform feature extraction. You can think of it in terms of classical digital processing techniques where convolutional operators have been used to extract specific features from images like edges and boundaries or correlations. The convolutional layer can be thought of as a way of learning convolutional operations to extract relevant features. Likely you will have to do some feature engineering beforehand though as you may want to preprocess by normalizing or downsampling etc, but it depends.