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.