I don't get it - the "problem" with the client/server example in particular (which seems pivotal in the explanation). But I am also unfamiliar with zig, maybe that's a prerequisite. (I am however familiar with async, concurrency, and parallelism)
Example 1
You can write to one file, wait, and then write to the second file.
Concurrency not required.
Example 2
You can NOT do Server.accept, wait, and then do Client.connect, because Server.accept would block forever.
Concurrency required.
Example 1
You can write to one file, wait, and then write to the second file.
Concurrency not required.
Example 2
You can NOT do Server.accept, wait, and then do Client.connect, because Server.accept would block forever.
Concurrency required.