The needs for random numbers vary by application. I'm satisfied with the Johnson noise generated by a resistor, feeding an a/d converter, for testing audio circuits. I only need a sampling rate of a few tens of kHz. For applications like encryption, what's important is not just each number in isolation, but ensuring that sequences of random numbers have required properties that make it hard to guess a number in the sequence from its predecessors. Or to guess that two different sequences come from the same computer. You might need to generate sequences at a rate that make it hard to do with purely physical random sources.

And you need to be assured that a sequence meets those needs without being able to test it to exhaustion, which means it has to be provable from inspection of the algorithm and original sources of entropy. At least, provable enough for the application. ;-)