The better players are at understanding how their opponents think, the more successful they will be. Experts are very good at characterizing their opponents and exploiting weaknesses in their play, and knowing when they do or do not have the best hand. They often try to put an opponent on a certain range of hands (guess the cards they hold) based on observed actions. It is important to note here that, if a player wins a game uncontested (no showdown), they do not have to reveal their cards. The showdown (exposure of an opponent's hidden cards) gives away information that can be used with the betting history to infer the decision-making process.
To take a less specific approach, one can estimate probabilities for a generic (or ``reasonable") opponent. However, observing an opponent's play may give you useful information that allows you to bias the probabilities, allowing for more informed (and more profitable) decisions. For example, an observant player is less likely to take a bet seriously from someone who bets aggressively every game. Good opponent modeling is vital to having a good estimate of hand strength and potential.