While you develop web applications, you will always end up doing the validations for your UI, both at the client and server sides, as the client-side validations are required for better UI experience and server-side validations are needed for better security of the app. Think about two different languages in action: you will have the same logic implemented in both the server and client sides. With Node.js, you can think of sharing the common function between server and client, reducing the code duplication to a large extent.

Ever worked on optimizing the load time for client-side components of your Single-Page Application (SPA) loaded from template engines such as Underscore? You would end up thinking about a way we could share the rendering of templates in both server and client at the same time; some call it hybrid templating.

Node.js resolves the context of duplication of client templates better than any other server-side technologies, just because we can use the same JS templating framework and the templates both at server and client.

If you are taking this point lightly, the problem it resolves is not just the issue of reusing validations or templates on the server and client. Think about an SPA being built; you will need to implement the subsets of server-side models in the client-side MV* framework also. Now, think about the templates, models, and controller subsets being shared on both client and server. We are solving a higher scenario of code redundancy.