One example: input validation logic is something that is nice to run in the front end (for performance and immediate responsiveness to users) but that you also need to run in the back end for correctness.
Previously I've done this with 2 implementations (JS on frontend and Java on backend), but then keeping the logic in-sync is a nightmare. With a single language you can just share the library.
You are correct, though, the biggest benefit I see is not sharing code, but making it trivially easy for a front-end dev to add a small piece of backend code that they need without needing a back-end dev to do it, and vice versa. It just makes the overall team much more productive because there is very little "waiting on the back/front-end" to do it.
Oftentimes we'll have either team write up the schema for a new endpoint using the GraphQL schema definition language, then from that we autogenerate the TypeScript types, then usually the front-end team creates a simple mock in the backend so that they can fully implement the UI, meanwhile the backend team works concurrently on the real implementation. This process allows for much more parallel productivity than if, when something is broken or needed from the other team, you just have to submit a ticket and wait.
Previously I've done this with 2 implementations (JS on frontend and Java on backend), but then keeping the logic in-sync is a nightmare. With a single language you can just share the library.
You are correct, though, the biggest benefit I see is not sharing code, but making it trivially easy for a front-end dev to add a small piece of backend code that they need without needing a back-end dev to do it, and vice versa. It just makes the overall team much more productive because there is very little "waiting on the back/front-end" to do it.
Oftentimes we'll have either team write up the schema for a new endpoint using the GraphQL schema definition language, then from that we autogenerate the TypeScript types, then usually the front-end team creates a simple mock in the backend so that they can fully implement the UI, meanwhile the backend team works concurrently on the real implementation. This process allows for much more parallel productivity than if, when something is broken or needed from the other team, you just have to submit a ticket and wait.