[abatilo]

As opposed to keeping all of your servers independent of each other, super computers are used any time you want to pretend the entire computer is one computer.

In other words, they're used when you want to share some kind of state across all of the computers, without the potential overhead of communicating to some other system like a database.

Physics simulations and like, molecular modeling come to mind as common examples.

In the case of ML training, model parameters and broadcasting the deltas that get calculated during training are that shared state.