You want to hit the ball as flat as possible, this has three good reasons:
A flat ball is harder for your opponents to hit back. This has everything to do with the fact that the ball comes down on the feet of the opponents. Hitting a ball flat is also less risky because the swing on itself is a lot easier than slicing, lobbing etc. The better you get, the more you want to let your opponents walk over the court. If you’re able to hit a lot of balls without making a mistake, you’re more likely to win the point!
A flat ball will go down earlier than a slice ball and will therefor go to harder places for your opponents. If you place 5 slice balls during a long rally, it is likely that your opponent already made the point. If you hit the ball flat, you are able to win more rallies and you have more options to hit the ball to. It is also used to surprise your opponent because you can “hide” the swing much better than with other swings.
The same benefit you have with the lob. If you hit a flat lob, you have much more space and time to hit the ball well. This is a big benefit compared to the high slice. If you play this, you have to time and aim very well in order to hit the ball good.
A flat ball is easy to disguise. You pretend to hit a slice of a lob, but than you play a powerful flat ball out of nowhere. This is handy if you want to surprise your opponents.