I like Adam's (@adambene) answer. IMO, companies that are trying to build universal (isomorphic) SPAs will reuse a lot of code on backend and frontend. For example, Hashnode is a universal app. So, it's important for us to write components that can be rendered on both client and server. This is where you will need to reuse a lot of code between frontend and backend.
But when I used to build simple SPAs with Angular, I didn't reuse a lot of code on frontend. The only places where I reused code were form validations and other simple stuff. I don't think developers usually reuse a lot of code between client and server, unless they are building isomorphic apps.
@JanVladimirMostert Yes. I believe code reuse is minimal if you are not building isomorphic apps.