It's called methodology and documentation, feelancer. It makes a website portable, maintainable and accountable. There's simply too many companies belching out sites without a shred of documentation except for a bill. If you want to tell your "web designer" to pack his bags and get the hell out of your office you're stuck with a black box website that you are often better off chucking out the window and starting over than having to decipher how the hell the thing works.
It's about process and giving the power back to the person who pays for the site. Good method and good documentation does that.
It's also about quantifiability. Providing estimates and actuals for costs for each component for each phase. Its about tracking actual performance of the site once it's running and determining whether any benefit is being derived from each of the components of the website. Finally it is about comparing the cumulative costs to the cumulative benefits of the site to provide a conclusion as to whether the site is cost effective.
Yeah, it's all there in that one diagram.