Using martingale in forex is based in probability theory. The more equity you have the more chances to cover your losses and be in profit.
Wikipedia: “In probability theory, a martingale is a sequence of random variables (i.e., a stochastic process) for which, at a particular time in the realized sequence, the expectation of the next value in the sequence is equal to the present observed value even given knowledge of all prior observed values.”
Martingale is popular in gambling games and widely used in games. In modern casinos they change rules to avoid martingale system. They put limits on minimum and maximum amount to bet; also they put two green fields to the roulette wheel.