The slow flight exercise is normally a very useful way of improving a students aileron/rudder coordination, due to the fact the adverse aileron yaw effect is much more pronounced at low speed.
The slow flight exercise is certainly useful in preparing a student for flying a final approach, I usually find when slow flying hasn't been covered adequately on the final approach a student will undercontrol with the rudder and overcontrol with the aileron. If you consider a C152 is cruised at 90kts and the approach flown at 65/60 knots then there is a considerable difference in the handling characteristics at these speeds, hence the slow flying is an exercise in refined flying control required for flying a final approach.
If Ex10a and 8 part 2 are covered well it certainly saves time at the circuit stage.