(original: June 2012 - updated Aug 2016)
Basically it helps you develop in a methodical manor, and ensures your code is object-oriented and uses best practices; whilst at the same time allowing you to utilise the ready-to-use libraries/components ranging from: handling-dates to full-user-authentication.
But which framework? The most popular are Zend and Codeigniter:
Probably the best framework to use is Zend - however it is bloated in many senses [much like microsoft windows] but is the most widely used and supported of all frameworks; once experienced you will get work using it in the future:
JOB prospects: good number of vacancies with a reasonable salary/rate per day
see the zend framework jobs available with reed
Is a more compact PHP framework, and it also comes with its own authentication libraries:
JOB prospects: reasonable number of vacancies with a reasonable salary/rate per day (but less jobs and lower salary than Zend), see: CodeIgniter Framework jobs
So to conclude...
Zend is the best long term route; CodeIgniter is the closest contender and worth a consideration. If you deploy smaller simple apps then CodeIgniter will probably suit your needs, but if you want to use what is quickly becoming an industrial standard for the PHP framework then it has to be Zend.