To play winning poker, you have to be able to determine what hands are playable on the board as soon as the cards come out.  To be fair, it might not actually be important that you analyze the board in that split second, but you must be able to analyze it quickly enough that you won’t get burned by betting against a possible hand you missed.  let’s do a quick test on reading the board:  Qd, 10d, 4c, 8h

What is the best hand you can make on that board?  The correct answer is, of course, a queen-high straight.  To make that hand, you would have to have 9-J as your hold cards.  Let’s add a river card and see how that changes things.  The river card drops and the board now shows: Qd, 10d, 4c, 8h, Ad

Now what is the nuts?  If you said ace-high flush you are partially right.  Since the ace is on the board, the actual nuts is the ace-high flush with the king of diamonds in your hand.  Since anyone who makes the flush would have the ace in their hand, the next-highest card would decide a winner between two flushes.

This example shows how one card can dramatically change the value of your hand.  After the turn card fell, anyone holding the 9-J hand a great hand but once the diamond hit on the river, their once-powerful straight is vulnerable to anyone holding two diamonds.  By reading the board and spotting this possible flush draw on the turn, a smart player would be able to protect their hand with a strong bet.  Being able to read the board not only allows you to pick out the best hand, it also helps you make better-informed betting decisions.