Frequently, you will find yourself in a position such that you cannot get where you want to go without assuming a favorable placement of the other side's cards. It may be as simple as assuming that a particular card is in one opponent's hand.
Beyond that, carrying your assumption to its logical conclusion can produce results that you might not dream of otherwise.
Assume that the cards lie such that the contract can be made. The results of your assumption may critically affect play in another suit. For example:
Against 4 Spades, flanker leads the 2 of Hearts. You play the King from dummy. Jammer wins, cashes the Queen of Hearts, and shifts to the 3 of Clubs.
Since you must always lose a trick to the Ace of Clubs, you must play for the King of Diamonds to be on your right.
Had jammer originally held the Ace-Queen of Hearts, King of Diamonds and Ace of Clubs, adding up to 13 points, would she not have opened the bidding? So, you must play for the Ace of Clubs without the Queen to be on your left, by putting in the Jack or the ten.