The pro shop balls have a durability factor and will "wear out" after a few rounds. If you have more of the same type of balls in your "bag",when one wears out, the caddie will hand you a new one. If not, you go back to the freebies.
There is a bar on the bottom right of the play screen that shows how much durability the ball has. When it gets low the bar turns from white to red to warn that it`s almost spent. When you get a new one, it tells you on screen "equiped new ball" or something like that.
The balls have different wear factors shown on screen when your in the pro shop. Check it out next time your there. GL