Most of us already know the use of transaction fees, but I know for certain that Mr. Average Joe Blow that uses bitcoins will not understand the technical aspect of it.
And if you are Joe Blow, this is for you
: You do not need to pay a transaction fee if the coins in your wallet are "mature" enough. If they're not mature and it's a small payment, then you're at the mercy of the miners to confirm your payment if you decide not to pay the fee. SO PAY THE FEE.