Difference between usage of Dispatcher IO and Default

So basically they run on the same thread anyway. There is a real difference or in the end it won't matter each one to use?

Your quote from the documentation just details an optimization Kotlin introduced: when switching contexts between Default and IO, you don't pay for the expensive handoff to another thread; instead the thread itself moves to the other pool.

This doesn't affect the overall properties of the thread pools behind the Default and IO dispatchers:

• Default has a fixed size hardcoded to the number of available processors, because that's what makes sense for CPU-intensive tasks.
• IO is an elastic thread pool with a configurable max size. Threads in this pool are expected to spend most of their time in a non-runnable state, awaiting the completion of an IO operation, so it makes sense to have more of them than there are CPU cores.

The difference is that Dispatchers.Default is limited to the number of CPU cores (with a minimum of 2) so only N (where N == cpu cores) tasks can run in parallel in this dispatcher.

On the IO dispatcher there are by default 64 threads, so there could be up to 64 parallel tasks running on that dispatcher.

The idea is that the IO dispatcher spends a lot of time waiting (IO blocked), while the Default dispatcher is intended for CPU intensive tasks, where there is little or no sleep.