Modern applications are all interleaved. Take any UI application. Every seasoned UI developer know how it is hard to debug this subtle bugs when start to crop up when suddenly the events that were coming in one order start to come in the other order and break some assumption and invariants in the code. Developers think sequentially. Sequential code is the most natural one to write and understand, but you somehow have to be able to maintain this complex UI application where user does multiple things at the same time.