With classical computing, if you plug in a widget and that widget spits out a bunch of supposedly random numbers, then you have to take it on faith that the widget isn't cheating and generating highly biased output. (For example, a deterministic cryptographic RNG is almost maximally biased, but it's computationally infeasible to tell just by examining the output.)
With quantum techniques, if you are willing to plug in two devices and process their outputs a bit, and if you're willing to trust that the devices aren't secretly communicating, then you can get provable randomness. You don't even need a real quantum computer to do this. [1] You still need to trust that the devices aren't actively malicious, but you do rule out accidental implementation issues that introduce subtle correlations between output bits.
If you are using an actual quantum computer, then you can go much farther and generate uniform random bits directly and even, under certain straightforward conditions, prove that those bits didn't leak out via a side channel. This is more or less the same principle as quantum key distribution: if Alice and Bob run a QKD algorithm to generate a key (n uniform, independent random bits), they are effectively running a protocol that verifies that no side channel leaked the bits. A malicious eavesdropper is a form of side channel, so those get ruled out as well.