The word balancing (Eng. range balance) is not that old in the vocabulary of most average players. Not so long ago, balance wasn't even discussed. With its arrival in poker terminology, almost everyone concluded about the undeniable value of balance.
There is an obvious connection between imbalance and vulnerability. But being vulnerable is not the same as being completely defeated. Remember, to attack, you must open a gap in your defense. So, to completely break something, you must also be vulnerable.
For example, your opponent rarely bluffs in a certain spot. To beat his strategy, you should never make a hero call because he almost always has something. This might seem like a natural adjustment, but in reality, game theory says you need to become vulnerable to defeat your opponent. If you now fold 100% of bluff catchers, you allow your opponent to beat you 100% of the time when he bluffs. Keep in mind that it's okay to be vulnerable. You need to be so to play well and adapt to your opponent.
You are probably well acquainted with the term optimal play, but optimality has a very specific meaning in game theory. Being optimal means maximizing your exploitation of your opponent (in other words, maximizing your EV advantage). Playing optimally does not necessarily mean playing an unexploitable game, as we have already discussed that you cannot defeat something if you are not vulnerable yourself. A perfect mathematical poker program, to defeat an artificial opponent (computer), must choose an optimal and balanced strategy, while against a human opponent, it must choose a suboptimal strategy.
You might wonder why players say you need to be balanced if, in reality, to be unbeatable and perfectly balanced, you should never choose an optimal strategy? Why then is balance so important if it is never optimal?
It is important because poker is never played with a perfect mathematical program. Poker is played over time. Instead of looking at poker synchronously, as a moment “frozen” in time, let's look at it asynchronously, as it changes over a certain period. If your opponent makes a bet on the river with a perfectly balanced range, such that our EV in every hero call is zero, it is fine to never make a hero call or to make a hero call 100% of the time. However, such an action does not take into account changes during the session. As soon as we start responding with a hero call 100% of the time, at some point, the opponent will notice and react by changing his unbeatable strategy to a vulnerable one, stopping bluffing altogether. Such an action would make our asynchronous EV dramatically negative and the opponent's positive. His unbeatability, balance, created a fortress from which he can attack whenever he wants, deciding when and how to tilt, becoming vulnerable precisely when he has gathered enough necessary information.
This means that balance and unbeatability are strong defensive strategies. It is a way to safely invest your time until you find the optimal strategy.
Unbeatability is also a valuable defensive strategy because human attention is limited. People usually make the same mistake – they conclude that in the future, you will react to information exactly as you do now. The opponent should break everything down into your mistakes, emotions, the possibility that you simply missed the necessary information, etc. Unfortunately, when looking at other players, we often attribute everything to their mistakes, completely forgetting to think about ourselves.
The point is that during a poker session, you will encounter a huge amount of poker statistics and your interaction with the opponent. You will see his 
preflop raising range, 3-betting frequency, his flop c-betting percentage, different ranges of hands with which he c-bets and with which he checks back, you will see how often he checks monster hands, and the list can go on and on. In reality, although all these numbers are right in front of you, you do not perceive them all at once. Your attention is limited. There are certain things you can focus on and use the necessary information to play, while everything else just slips by. And although a player chooses certain things to pay attention to, he does not always act accordingly.
Let's remember the poker ship metaphor: it would be foolish to open gaps all over the ship. Even if you have a cannon for each of them, you can only focus your attention on a certain number. After all, you are the only crew member on this ship, so you need to have a plan. Keep your ship strongly fortified where your attention does not reach, and in places where your attention is active and constant, open up for attack so you can strike yourself. Just because your attention is limited, it is extremely valuable to play a defensive strategy that makes you unbeatable.
