Car seat canopy - necessary?

Do you use your car seat canopy?  What's its primary function?  Weather gaurd?  Stranger touches shield?  Or is this just one of those extra things you register for, but it sits around and doesn't get used?  It's on my registry checklist, but I'm trying to keep it minimal - I don't want stuff I don't need.  On the other hand, if it truly is useful, I'd like to add it.  Would love to hear from mammas!