It's a linear (antilog) to log voltage converter. It has a range of 80db which is 10,000:1. Suppose you wanted to measure a voltage that could vary from 1 unit to 10,000 units. And you also want to plot the changes on a chart recorder (graph paper). Well, using a linear graph, would require at least 10,000 lines for an accurate plot. That's some big paper! But if you convert that voltage to a logarithm first, the change is only 80db. Now you can use graph paper with only 80 lines on it. Much more manageable.
The formula to convert linear voltage to log voltage is db=20log(E1/E2)
I doubt you will ever have a need for it. It needs to be in a laboratory along with chart recorders and other fancy scientist/engineer stuff.
Here's a link to a real world application and description of your converter. Look at pages 9-10 only.http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1968-10.pdf