I think you must understand how and where the rubber will meet the road (strings meeting the ball):
For topspin: You peel an orange from 6 to 12 o' clock positions causing the ball to rotate forward-upward. The racket-head at contact goes more on a vertical path and less on a linear path. The topspin shot clears the net with high margin.
For flat: You smash the orange .. the strings go through the ball more on a linear path (as if the ball will pass through the head of the racket). Flat shot clears the net with less margin.
Topspin keeps you more in rally situations. Basically it is a safe shot. A player like Nadal could be very aggressive with topspin.
However, when the winner presents itself you flaten it out and hit a flat winner to a nearby corner.
An experienced player will mix his shots with topspin, flat and slice to offer a deadly cocktail to his opponent.
You must have all the shots .. topspin, flat, slice .. to become a complete player.