Ask HN: What is the point of parallelism when you can multiple instances of app?

For some reason I always assumed that you could only run one app per cpu core. So if your language didn't support multi-threading, you have to find multi-core cpus. And scaling it would require that expense. But.... that's not the case, you can run as many as apps you want. In hindsight, of course, I used to have pentium cpu with single core and OS can converge instructions from multiple ongoing tasks and give impression of parallelism.

But then my question is what is the advantage of parallelism. Why not simply just run multiple instances and keep data outside of app in some sort of shared memory?