Partly if we talk a Pentium 4 CPU up to HT (single core), still it would stop the main thread run the other and then resume where it was. So technically it's the same but different.
No, because the scheduler guarantees each thread X time units (probably microseconds, I don't know how the Windows scheduler works in detail).
When you spawn another thread, you have two times X time units (well, probably the second thread won't get the same time as the main thread and there is also the thing with priorities).
So, even with just one CPU Core you can still boost up your game by moving stuff into a second thread.
