what do block off plates do exactly?

 

You put them on if you dont wish to have any emissions at all anymore. Better responce....

 

they eliminate a lot of functions that are seen as "useless" by a lot of owners...such as that damn accelerated warmup...plus if you are a wrenchhead and do all your own tuning and plan on getting under the manifold a bunch, they make it a LOT easeir to deal with by reducing the hoses and such...