Why are the mangroves important?

Because not only are they endangered compared to a lot of other semi tropical/tropical flora, but they're a vital part of the prevention of beach erosion. The roots hold the soil/sand in place along beach shores.

A while back, it was not thought that they were important, because a Plant native to Brazil call the "Brazilian pepper tree" was almost the same thing and could grow along shorelines of salt water also, but in recent years it has been discovered that Brazilian pepper is a nuisance. Not only is it poisonous, but its destructive and illegal to plant now because of it.