Isn't that a different idea than a timetable? "We're aiming for one train every 10min" and "there's a train at 12.10, 12.20, 12.30" are similar, but not quite the same. You can get your approximation without a timetable though.
Tracked vehicles require centralised traffic management and scheduling as they cannot casually overtake one another as street traffic can, and setting switching points and clearing control blocks is required for safety.
Passengers transferring to other lines or transit modes may also appreciate predictability.