CheckSite automatically checks all pages of a website for a number of potential problems, and reports these. This makes it much easier to test a website, and because the process is automated it can easily be repeated.
Although popular opinion says the contrary, writing good HTML is actually not that easy. First, the page should contain no errors, which is rarely the case. But even without errors, the page might still not display consistently across browsers, it might be poor from a SEO (Search Engine Optimization) perspective, or it might cause problems for visitors from non-Western countries.