What is the function of a barrier island?

To put it simply, the main function of a Barrier Island is to protect the coastline, meaning that it serves as a shield for the coast, taking all the damage from the shifting tides and waves, and the violent waves that would bring severe damage to beaches.

Just relate to common knoledge things, like football. I forget the exact term, but it's when a player gets the football, and the other defensemen surround him in the shape of an arrow. They take all the hard hits (Barrier Island) so the guy with the football (Coastline) doesn't get tackled.

Another function of a Barrier Island that it harbors many types of wildlife that is not commonly found elsewhere, which is why bulding condos' or houses or even entire towns on a Barrier Island is not good for the surrounding environment. But do people still build on them anyway? Yup. Welcome to the 21st Century.

Well, I hope I was able to help you out in some way or another!