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.

Today most/all CPUs is multicore like Dual, Quad or i7 (8 cores) when kicking of a new thread this will take advantage of the cores still idle or low utilized (and run in parallel). Still the main thread will run at full speed so to say. Multithread make it possible to lift out heavy or slow processes from the main thread to run on an own thread and utilize the full potential of multicore CPUs if available.

Of course this also turn around how you program since you must make algorithms to take care of scheduling and keep track of all threads so you don't start to use results that still is not calculated or overwrite results that not yet has been used, but that is a totally other topic.

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.

True, but to a cost for other running applications, OS core functions and services since there is still only one CPU to spread the workload on. My answer was a general on benefits from multithreading with multicore CPU since that is what most people have in their PC nowadays. Nitpicking on hardcore details was never my intention.

I remember reading that multi threads within GS are benificial when using them
multi media functions.

Are you talking about this thread: http://www.opserver.de/ubb7/ubbthreads.php?ubb=showflat&Number=341634#Post341634 ?
If so, this reads more like "we are using already multithreading for media functions" and not like "media functions would be faster if you knew how to spawn a thread."

Btw, not only media functions can be faster, everything that isn't time critical can be boost up. Eg. HTTP Requests or every other operation that uses the network should run asynchronously in another thread. Also Pathfinding could run in a second thread.

@JustSid Your right sorry about that.

As quoted from jcl: The engine itself can not use multiple cores because the main engine functions must run in a certain order.

But side functions, such as video or audio streaming, use multiple cores. You can also use them the way described above when you have time consuming functions.

so tracing wouldnt work in a new thread?