Tied with two lines as you describe, when a boat passes you surge forward. This means the stern lines comes tight and holds the back in, but the fore line slacks slightly allowing the bow to move out. As the surge reverses you get the opposite reaction.
With springs, although you still surge forwards the bow and stern still remain tight to the bank, and as you surge back the same reaction. To explain further, as you surge forwards the stern line will tighten, but so will the forward spring, and then the converse.
So although the boat will still move slightly forwards and backwards you will no longer have one end or the other swinging out and bouncing back in.
Although you may think your two lines are as tight as they can possibly be, the water level will change slightly as a boat passes which will immediately mean they are less tight (or more, depending upon the relative levels)
Tam