Designing and coding CSS based templates is not the easiest thing for many people. To top it many designers do not have much time to spare from their complex design work to learn it.
For many designers Photoshop is the ultimate tool and they can create numerous design using it, every design they create results in a PSD file which is a format in which Photoshop stores the design. The problem arises when a designer needs to convert the final PSD file into a valid XHTML / CSS template.
Like they say for every problem lies a solution and psd,css creator,xhtml creator,psd to css