Between Fou-Lu's and my own examples/explanations, as well as greeninho's clarification on the return types (ie void, int, double, etc) I don't think there is anything more we can do to make it "more" clear... I think what you should do (because at this point I am guessing you are understanding what we are saying, but the "ah-ha!" moment hasn't hit yet) is just take it on faith that good coding practice is done this way. And albeit a piss-poor explanation- for now all you really need (since you are a "newbie") is...
do it this way because people in the industry who have been doing it, and developed it said so.
Eventually you will get to the point, and level of understanding, that this will all make sense. You will see where/when to make thing public or private; you will notice the benefits. It is all too much to try and understand "everything" at once.