Same language, in theory, is great. Reuse structure definitions. Reuse rendering logic or even validation logic (perform checks on both, but make it easy to get into client-side). In general, keeping things "in sync".

Also can make it easier to write in SPA style but offload rendering to the server when you need it (particularly first-page or reloads).

Whether or not tooling is good enough to allow this (either with JS or compilers) is another issue.