I have never used php classes and object with it. All i know is writing functions and have always used simple php code. Is there any good reason for me to use a php class?
This depends from your project. If you need simple script or simple website - so you can easy use only functions and be happy. If you want to develop really complex website, or use any PHP framework - you must learn Object Oriented Programming on PHP. You can read detailed about classes here http://www.php.net/manual/en/language.oop5.php
Yes. OOP allows you to more easily reuse and organize code. Learning classes expands your abilities as a coder. Yes, but so can procedural coding; It just depends how you are doing it. The primary reason for OOP is to keep your code organized and reusable. Classes are useful when working on a project with multiple developers as well. There is a lot you can do with classes, so I recommend learning them well, especially if you ever intend to use a framework.
To be very very simple. A class is a group of a load of functions. I learnt OOP a while back while writing a custom CMS. Well worth it.
All of the big CMS systems and stuff you'll see around the net are OOP and built around classes.. These forums (vBulletin) being a perfect example..