The project he worked on was a real-time remote procedure call which gave him experience in computer networking.
When stack-based procedure calls were first introduced, an important motivation was to save precious memory.
Communication between processes largely consists of signals and procedure calls.
This is a remote procedure call and task distribution pattern.
If you want to "nest" the tests, you should use a procedure call.
Some of these registers are not saved or restored on each procedure call.
This construct just looks like the parameter pack of a procedure call.
The idea was to make one particularly common instruction, the procedure call, extremely easy to implement in the compilers.
Thus the common procedure call does not have to interact with main memory, greatly accelerating it.
The call is a local procedure call, with parameters pushed on to the stack in the normal way.