For advanced poker players, who are often up against other advanced players, it is important to balance their range. Balancing your range is a process where you manipulate the frequency with which different hands occur in the range. The goal here is to stay unpredictable for your opponents. This means that there may not be any hands in the range that occur much more frequently than others.
Back in the day, online poker players would only re-raise very strong hands, such as QQ, KK, AA and AK. Since this is a very small range, strong players could perfectly adapt their game and fold other, medium-strength hands. Over the course of time, some players started 3-betting a wider range of hands in order to exploit their opponents’ weak folds with hands such as JJ or AQ. From that point on, a 3-bet was no longer a reliable indicator of a big pair; players would now even re-raise hands such as 56s to expand and therefore balance their range. This in turn forced the opponents to adjust their ranges as well. They now had to play more than just their premium hands, as otherwise they would have folded the best hands too often.
Good players always try to put their opponents on a range. That's why it is also important postflop to balance your range in order to remain impossible to read. If you could be holding a variety of possible hands in a given situation and your opponent can't make any accurate assumptions about them, you have successfully balanced your range.