An opponent may be nearing the completion of a meld with those particular cards if, for example, they routinely draw cards from the discard pile rather than the stockpile. Understanding this enables you to mrammy novelodify your own approach appropriately, possibly by keeping some cards that could finish their meld or by throwing away less valuable options. Also, psychological understanding of your opponents' inclinations & routines is just as important for effective anticipation as observation. Depending on how comfortable they are with particular card combinations or strategies, some players may play in predictable ways.