Object Oriented Programming question

Hi, I've been learning about object oriented programming for the past few weeks and I've understood it pretty well, but I have one question. Usually with PHP scripts I make, all the functionality for a specific page is in the actual PHP file, and I'd use PHP functions in a separate directory which would be included in whichever PHP file needs specific functions I have created. The functions would be for the specific things in my script, such as validation checks, functionality that will be used/repeated a lot throughout my script, etc. What I don't understand about OOP is what its primary purpose is for. Do I use OOP for all the functionality of my application, in separate directories, and include these specific class files and call the methods to complete specific functionality needed for whatever PHP file I'm working on, or is OOP used for specific functionality like I would with functions? Essentially what I'm asking is what is the primary purpose for OOP? Hope you understand. Thanks,