I often find the differences between State Machine and Sequential workflows much like those of event-driven programming with old control flows. Back to old days when we programmed with FORTRAN, we often thought of how to control the flow of program by branch instructions to do a job. The flow of control often had only one or at most a few of predefined paths. We could plan for these paths with the IF THEN branching instructions. Event driven programs are not so. Who can imagine the paths of instructions when someone uses Word?
State Machines are type of event-driven workflows most suitable for situations when it is hard to draw all of possible paths of a process. This is why State Machines provide a more flexible approach in programming Business Processes. It’s a pit SharePoint focuses on Sequential workflows.