Having a good sense of design is important for web professionals who fall more into the "designer" category. There is a lot more to web design than simply knowing which colors look good together. You must be knowledgeable of the elements of design as well as the basic design principles as well as typography best practices, how to use images, layout principals and more. You also need to have an understanding of how real people will interact with a design so that you can make the right decisions to help meet the needs of that site.