Doing one thing on multiple threads is very much possible. Threads enable (simple) shared memory multi-processing, which is the most efficient version of multi-processing. You can theoretically get something similar with different processes and memory-mapped files, but multi-threading is much easier in general.

For example, if you want a high-performance web server, it's much more efficient to serve requests through multiple threads (preferably also using efficient IO) than it is to spawn different processes for different requests. I imagine you can coordinate different processes in similar ways, but again, it's much more effort, and either way, those processes are far too intimately tied to each other at that point to call them "different tools".