I believe the point is that you get a uniform distirbution of 1s and 0s after going through the PRF irrespectively of the distribution of 1s and 0s in the input

I believe the point is that you get a uniform distirbution of 1s and 0s after going through the PRF irrespectively of the distribution of 1s and 0s in the input. This is certainly a desirable property for a hash function.