I'm attempting a full-stack program for the first time and am currently following the initial Next.js dashboard tutorial and have a question regarding React Server Actions. I'm still trying to figure out the purpose for some of the features included in the React framework. React Server Actions can be imported into both Client and Server components, but to my understanding Client Actions cannot be imported into Server components. It makes sense why you would want to mark a file with a 'use client' directive - to specify specific things for the client to load rather than the server.
Because Server Components are default, what is the advantage of defining a 'use server' directive and why is this something I need to define? As my title asks, why can we import Server Actions into Client components but not the other way around? Do we solely use API calls or hooks to communicate with the server?