V8 was designed to be multi-tenant. It was designed to run the code from the many tabs in the browser in isolated environments and within a single process.
Node.js and similar runtimes were not designed for multi-tenant
Isolates are lightweight contexts that group variables with the code allowed to mutate them. A single process can run thousands of Isolates, seamlessly switching between them.
Isolates were designed to start quickly and to not allow one Isolate to access the memory of another.
Isolates can start around a hundred times faster than a Node.js process on a local machine. Isolates consume an order of magnitude less memory than that Node.js process.
Isolates start in 5 milliseconds.