In table tennis, it's good to keep moving the ball around to make an opponent move. You should play all three spots - wide forehand, wide backhand, and middle (roughly the opponent's elbow, the transition point between forehand and backhand). However, sometimes you want to go to the same place twice. Here are a few examples.
You've just blocked the ball to the opponent's wide forehand. The opponent had to go out of position, but made a somewhat aggressive topspin return from his wide forehand. After the shot, he began to move back to cover his wide backhand. Most players try to take advantage of the opponent being out of position by going back to the backhand. This will often work, but it's often better to go right back to the forehand. The opponent is moving in the wrong direction, and will likely have trouble covering this shot effectively a second time in a row.
Suppose your opponent has a strong forehand but weak backhand. Let's say he covers two-thirds of the table with his forehand. If you play to his wide backhand, you'll get his weak backhand. It might be tempting to then change directions and go somewhere else, and that often works - but often it's better to just pin the player down on their weak side, and not change directions until you see your opponent out of position, perhaps edging toward his backhand side to use a forehand, thereby leaving the forehand side open. If you change directions while he's in position, you are just giving him his stronger shot - and if he's smart, he'll just wait for that shot if you are going to give it to him. (You might consider going to his middle, if you think you can do it without him playing a strong forehand.)
Now suppose you're playing a chopper. You've made a good attacking shot, but the chopper chopped it back. You did a drop shot, the chopper ran in and pushed it back, and then quickly stopped back for your next attack. Do you attack again? Only if the opponent is too close to the table, or if his push was weak. If the opponent is stepping back to prepare for your attack, why not do a second drop shot, and catch him going the wrong way? Most likely you won't get an ace, but you'll not only get a relatively weak return from a lunging opponent, but your opponent will probably now be jammed at the table, unable to get into position to chop your next ball. Easy point!
Finally, you've smacked a strong shot right at the opponent's playing elbow – usually a player's weakest spot. The opponent manages to make a return, but not a particularly strong one. You get ready to attack his weak return. Why not go right back to his middle again? If you go to the corners, you might give the opponent and easy forehand or backhand counter. By going to the middle again, you can catch him again. Since his previous shot was weak, he's unlikely to be looking to counter-attack from the middle since he'll more likely be in a defensive position.
So while it's important to move the ball around and play all three spots, it's also important to also know when not to move the ball around.