why do we need functions in swift, and what is an easy to understand definition of a func, or function?

A function is simply a convenient way for you to be able to write a piece of code which carries out a task and allows you to reuse that code by calling the function from different places in your program.

In short, due to DRY (Don't Repeat Yourself) principle. would you prefer to write the same code 100 times?

In the Swift programming language guide, functions are defined as: "self-contained chunks of code that perform a specific task. You give a function a name that identifies what it does, and this name is used to “call” the function to perform its task when needed".

You don't have to think too hard every time you do some mundane task, like making a sandwich, because your brain has automated it. It's the same with functions. They automate a particular task that you will need to preform several times over without thinking too hard, or taking up too much space as chunks of code.