Nothing beats the flexibility and control you get with hand-crafted static websites, especially when it comes to SEO. But if you've made and maintained a few static websites, you quickly realize that it has its limitations.
When you hand code HTML on a page by page basis, you end up creating duplication, because a lot of elements on each page (like headers and footers) will be common across the whole website. Duplication is bad because when you want to change one of these elements, you have to do it manually on all pages that contain that element.
Mixing structure and content
Static websites mix the structure of the page, with the content of the page. The structure of the page is the hierarchy of elements that constitute that page, and in most cases, is common across a lot of pages. The content on the other hand, varies on a page by page basis. Most of the duplication on static websites is related to the structure.
Dependency on multiple tools
When you manage a static website, you are dependent on a lot of tools, like ftp clients, ssh clients, etc. The absence of any one of these tools, for example, when you are on the go, makes updating the website very painful.
A CMS can solve these problems with:
Templates are a way to eliminate structure duplication by extracting the common structure of pages in one place. This way, any changes to the structure is limited to the template alone, effectively separating the structure of pages from their content. Templates can also be used to share content across pages to eliminate content duplication.
Browser based editing
Browser based editing allows management of the website on the go. Since all changes are done directly on the live website, it helps to have versioning, to be able to rollback to a previous version in case of errors.
Rewritepad is a simple hosted CMS that solves just these problems while still keeping it as flexible and simple as hand coding static websites.