I'm only an ardent viewer of crash investigation stuff, not a pro, but it seems to be a good way to show specifics of warning noises, engine sounds, unusual cabin noises (if relevant) and sometimes even structural failures happening over time in a more direct way from the cockpit voice recorder without sharing the actual "audio".

A distressed airplane makes a lot of noises which are very difficult for the human ear to pick out and identify. For example, multiple closely-spaced bangs or rumbling noises will appear distinctly on a spectrogram but will be very hard to hear.

Basically they were just trying to track down weird noises on the recording. They also went and recorded audio on a similar UPS MD-11 to try to identify the source. According to the NTSB:

"This high pitch ringing sound, primarily noted at approximately 6.35 kHz, occurred at 17:13:05.5 EST, shortly after the aircraft rotated for takeoff, and continued with varying amplitude throughout the remainder of the recording. Additionally, a tone at about 2.1 kHz was present along with the ringing that could not be identified."

Where else would they get sounds from inside the cockpit that weren’t transmitted on the radio?