I think web designers make the graphics for buttons and headers, and backgrounds, and logos.
as a primary task.
secondary tasks may involved filling out information, specific to the clients needs. The information the client gives you being inserted on the correct pages.
other tasks may be creating a blueprint for the web developer to construct. as all the layout for a website is coded in by the developer. It is not built like legos by the designer.
The Blueprint should be given by the client, but it seldom is. The developers usually go with a typical reused format they are used to. But a designer could take what the developer does half assed to a whole new level to make it more original. A developer can rearrange everything, but without graphics he is kinda limited.. especially with new Mobil phone accessibility. Every website is coded to be compatible with a phone.. which FAWKS up everything. makes it easily 3 times harder.
By using graphics you can float things, and simplify the process of computer to phone coding.. and that makes it easier on developers.