There's something that really tickles me about casting a big actor in a faceless role. The actors must get a kick out of it (or else why would they do it?) and it's a fun easter egg for audiences. It's also great fodder for a list like this. 'Hot Fuzz,' as well as anything created by Edgar Wright, has the perfect type of humor for this type of casting to work, which is why they put Cate Blanchett in the role of Janine, a character we never see outside of a crime-scene-clean-up mask.
Just because she's wearing a mask doesn't mean she doesn't have a crucial role. Her role is extremely important as a motivator for the main character. That's why it was smart to cast such a great actress, but they could have cast someone equally skilled but lesser known, so the joke isn't lost on us.