The process of wireframing is used to determine where on each page your content will be placed. Creating a wireframe prior to designing your page allows you to logically place your content without having the design elements interfere with the most important part of your site; the content.
Since a wireframe is a rough layout of your content elements, it is useful to create this in the same software that you use to design the site. Once you are done with the wireframing stage you can use these files as the first steps in the designing stages.
Below is an example of a wireframe:
