When results must be verifiable. Most Monte Carlo simulations use a seeded PRNG so other people can test and re-run them to validate the results. Many of today's video games make use of a seeded PRNG as well, e.g. The Binding of Isaac offers randomly generated dungeons but also shows you their seed values for easy sharing amongst players. Mersenne Twister was never cryptographically secure, it was just relatively fast, very fit for seeded pseudo-randomness. And PHP choose to break backwards compatibility.