You may have more than one 'First Call' or 'Return Visit' on one evening.
The lessons are to distinguish between those. The S-89 does not have a space for the lessons, this is purely added to make clear which assignment is meant if there is more than one of the same and not to inform of the lesson.
Of course that distinction could be made in a different way, but this is how they programmed TB...
If you use the 'copy assignment' button to send the assignment digitally, rather than the S-89, it will have the complete description plus lesson number.