Watching your opponent’s moves and tactics is extremely important in the game of Rummikub, for both offensive and defensive reasons. By watching the manner in which they play you are learning all of the patterns that they use.
Most average players and even some good players unfortunately tend to follow the same patterns. For example, they may consistently discard form their highest unwanted tiles to the lowest. If a player has been discarding 10’s, 11’s, 12’s, and 13’s and suddenly discards a 6 then you know something is up with their pattern of play. They may be holding melds and combinations in the 7 to 9 range or they are fishing for a 6 of a different color. It could be that both are true.
If they had been discarding in a more random pattern, then you would have a harder time figuring out this information. Another pattern is that they may always play their melds on the rack rather than the table. If they suddenly open early on the table, you have to assume that they are holding an unusual rack and are probably looking for other players to meld early on so that they can play their open melds. Being aware of these patterns will make you a much better player and will increase your chances of winning overall.
At the same time, you should be playing your own tiles in such a manner as to keep your opponents confused and guessing what you are going to do next. You must vary your style of play so that your own patterns are less obvious to your opponent. This does not necessarily mean playing each hand differently. In fact, it is more effective to play consistently for a while and then change tactics suddenly.
It is also good for your own playing since it forces you to concentrate on all the aspects of this highly strategic game. Under pressure, even the best players tend to fall into the patterns that are most comfortable for them. If you can break out of this, you have improved your playing by another measure, and improved your chances of being a consistent winner in Rummikub.