When publishing the app, a target architecture needs to be specified, because the compiler will create an architecture specific stub loader for the project (in Windows, the "exe" file).

In .NET Core, all C# projects (irrespective of whether they're applications or libraries) compile to .dll files. These .dll files are platform independent, or AnyCPU (of course unless they contain code that would only run on certain operating systems, such as P/Invoke calls). The applications compiled this way can be run by using the dotnet <DllName> command.

Now this is not convenient, because people expect an "exe" file to start a program with. This exe file (or an equivalent "no-extension" file for unix) is created by the publish command, in addition to copying everything together. The exe file with the same name as the main .dll is the stub-loader that basically calls dotnet <myapp.dll>, even if publishing a framework-dependent application. That's why even a program that is targeting AnyCpu needs a platform-dependent publish step.

[Note: On Windows, the exe file might already be created in the build step, as VS uses it. But you'll notice that it always has about the same size, for all your apps.]