What protects the coast?

I don't really know why some coasts are not protected, but my guess is that it's expensive most of the time to put up sea defenses... Some may be missing defense because it's not eco friendly (hard engineering, that is) and not very attractive. This lowers the value of beaches. Then there are some beaches on small islands or isolated coasts that maybe don't really need the protection because either no one cares or there's no need for them. Isolated Coastlines don't really need them...

Anyway, this was my geography homework question and i think I may have answered it... Maybe... Wtv.

Hope this answers your question!

- Stella