Max Supply is the best approximation of the maximum amount of coins that will ever exist in the lifetime of the cryptocurrency.
Knowing the maximum amount of coins that will ever exist in the lifetime of the cryptocurrency can allow investors to estimate possibilities of price dilution from current values.
Some coins, for example stablecoins or Doge, don’t have a hard-coded maximum supply. Like fiat money, it theoretically can be created an infinite number of times, and its ever-increasing popularity means that there's no telling how many total coins will be in circulation. That is why it is impossible to measure “Maximum supply” for such coins.