Its sort of like college, I suppose. Those guys can go through all the continuing ed and testing to get the paper, or to learn something new to improve their skills.
The theory is that the tests ensure they know their stuff. And Continuing Ed makes sure they know the new stuff regarding environmental issues, water conservation, etc.
It may not be 100% perfect, but in most cases it should show a greater breadth of knowledge and skill. I know the Audubon program recommends you hire certfied supts. as do many clubs. At most places, it might be a distinguisher, but these days, if it carries another $25K in salary, it might distinguish the wrong way at some cash strapped places. but, again, striving to be the best and continualy improve is generally a good thing for any professional.