Having good line speed is very important when salt water fly fishing. Dealing with the wind is vital. Your ability to control what path your line travels in the air when presenting your fly will help you accuracy big time. And accuracy is HUGE when sight fishing for permit, bonefish or tarpon on the salt water flats.
You have to battle for complete control of your line in the air against the hard tropical winds. High line speed helps immensely.
Watch this killer explain and demonstrate video on high line speed and take this with you to the flats.
To learn more about high line speed and other technical salt water casting check out The Bonefish School. It's a great way to learn the many different advanced salt water casting techniques needed to tackle the salt.