Explain to an idiot what spring lines are for

[ferrailleur]

Don't understand the usefulness of spring lines when docked at a slip. I read many manuals and asked many sailors..... No cleat cut/definitive answer. I still don't understand. Am I missing something big or an I a total idiot?
Don't hold back your comments please.


My docking situation: I have two bow lines (secured on pilings) and two stern lines cleated on the pier.... (reverse situation if berthing the other way).

The boat can move a little fore and aft and side to side to adjust with a max three feet tide.

How in the world a spring line going to help with fore/aft movement of the boat which is already tied four way fore and aft?

Thanks