Sequence-to-sequence models have quite a few practical applications.
Perhaps the most practical application is machine translation. We can use machine translation to take a phrase in one language as input and output that phrase in another language. Machine translation is an important service that we depend on more and more. Thanks to advances in computer vision and machine translation, we can listen to a language we don't know, or look at a sign in a language we don't know, and have a pretty good translation almost immediately on our smartphone. Sequence-to-sequence networks really have gotten us very close to Douglas Adam's imagined babel fish from The Hitchhiker's Guide to the Galaxy.
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