the ability to hit both with spin and flat is very useful in so many situations:
- mixing up spins/flat during long rallies in order to force errors by mixing heights of the ball low, waist high & at the shoulders (slice, flat and topspin)
- using spins to hit high % conservative shots during rallies, i.e. can hit a meter or so (depending on spin and court surface) away from the baseline but stay neutral in the rally. allows you to recover and start a new sequence of shots.
- hitting passing shots that dip rapidly below the net.
- drop shots can cause more trouble for your opponent when they have a bit of side-spin.
Disclaimer: I'm NOT a coach...
Real tennis: https://www.youtube.com/watch?v=yDqnkLJ9BtM