Because one of them can be slower than the other? Let's assume random.org gives true quantum randomness. Let's assume it's acceptable for my program to make an

Because one of them can be slower than the other? Let's assume random.org gives true quantum randomness. Let's assume it's acceptable for my program to make an api call to random.org to get some random numbers. That doesn't mean I want to go to internet every time I need a random number. I'd much rather get something like 256 random bits from random.org and create the rest with Mersenne Twister which has an insanely large period.