Threads in independent situations
As discussed above in the Where a thread is a good idea section, threads also find use where a number of independent processing algorithms are occurring with shared data structures. While strictly speaking you could have a number of processes (each with one thread) explicitly sharing data, in some cases it's far more convenient to have a number of threads in one process instead. Let's see why and where you'd use threads in this case.
For our examples, we'll evolve a standard input/process/output model. In the most generic sense, one part of the model is responsible for getting input from somewhere, another part is responsible for processing the input to produce some form of output (or control), and the third part is responsible for feeding the output somewhere.