What is the spectrogram of an impulse response used for?
I think you are thinking of waterfall plots and/or cumulative spectral decay plots.  They are spectograms in the widest sense of the word since their windowing function tends to be exponential.  They are popular for plotting the response of speaker systems as they show group delay for various frequencies a lot more, well, graphically than the impulse response itself or a magnitude/phase plot of the transfer function would.  Resonators can be used for straightening out an otherwise wobbly (for whatever unrelated reason) frequency response magnitude at the cost of introducing delays.

The resulting lack of "impulse accuracy" which leads to problems with sound clarity and spatial perception is not easily apparent with other representations.