SharePoint Designer 2010 is the website editing tool for SharePoint. And typically, as the name implies, it is certainly the tool a web designer would use by editing web pages, HTML, styling (CSS), and much more.
However, this chapter's focus is more on the use of SharePoint Designer 2010 for developers. This tool is a ground-up redesign from its 2007 predecessor, and it in fact ensures that the developer is a target audience for this tool.
SharePoint 2010 is, at its heart, a product that a developer should consider first for configuring the product to do what your users want, before resorting to a code (Visual Studio) solution. You have already performed some out-of-the-box configuration using "in browser" tools, and SharePoint Designer 2010 can take configuration to another level.
And, of course, SharePoint Designer 2010 requires a good exploration of its capabilities, so that you understand if some task would, in fact, require a Visual Studio customization.