Professionally written PHP applications do not require a framework. Frameworks exist for people that have been doing some of the same repetitive tasks repetitively and desire to change their own workflow.
You should start by researching the "MVC design pattern" in general (as it applies to many things outside of PHP) and get deeper into the concepts of Professional PHP before starting with such a large framework as Zend Framework.
Use a framework because you know you need to, and not for any other reason.
