Frameworks provide scaffolding that can allow you to develop faster/more cleanly. They often provide toolsets for both the UI components and the underlying database access.
For (very) small projects, a framework can be overkill, but often, it's helpful to provide you with a lot of reusable code.
Some notable frameworks for PHP :
Zend Framework
CodeIgniter
Symfony
CakePHP
Mojavi
A comparison chart.
A more detailed review site.