There remained the important verification that the spectrum was indeed that of free radiation. But there was a more important question.
If the radiation is fully homogeneous, so would be the charged medium in contact with it. Thus the neutral Hydrogen formed would remain a perfectly homogeneous free gas. The problem with that is that no stars or galaxies could form, no sun could exist, no earth, no author, nor you dear reader could exist.
Our picture of the Universe today based on extensive observations confirms two aspects. The existence of galaxies shows there was inhomogeneity in the original plasma. But the distribution of galactic clusters is homogeneous. In other words, inhomogeneities must exist at large scales. But the distribution of the inhomogeneities must be uniform, since they are only small departures from an overall homogeneous situation.
Classical analysis showed that the formation of galaxies can be explained
correctly if the extent of inhomogeneities was about 
in 
or ten parts per million. If extrapolated from the last scattering
epoch till the present epoch, this magnitude of seed inhomogeneities
was just right to form the observed galactic distribution.
The COBE satellite experiment was fitted with three instruments. Two of them are relevant to our discussion. One called FIRAS measured the entire spectrum as accurately as possible. Another experiment focused on measuring the departure from perfect homogeneity. Both experiments performed with flying colors.