It's for one server/IP serving multiple hostnames. For instance, the same physical server at 45.76.26.79 serves both www.lukeshu.com and git.lukeshu.com with the same instance of Nginx. Once Nginx decrypts the request, it needs to know which `server { … }` block to use to generate the reply.
With TLS+SNI, this is redundant to the name from SNI. But we had TLS long before we had SNI, and we had HTTP long before we had TLS, and both of those scenarios need the `Host` header.
With TLS+SNI, this is redundant to the name from SNI. But we had TLS long before we had SNI, and we had HTTP long before we had TLS, and both of those scenarios need the `Host` header.