If you serve your static resources through S3, you can share the same static resources among multiple app servers. Also, a proper CDN will serve them a lot faster than your app server through nginx can.
CDNs aren't necessarily faster because you have to have a connection to a different server. However the idea is that say you have jQuery loaded for a CDN, and so do a bunch of other sites (so, same URL), they will already have the thing cached when they load up your site.
