Based on posts here and TGP, it seems many people have a less than positive opinion of Pure Cab. While I didn't intentionally set out to analyze what Pure Cab does to the frequency response, I just happened to look at a spectrum analyzer while adjusting Pure Cab and noticed something interesting. Here's what the frequency response looks like with and without it:
Without Pure Cab:
With Pure Cab:
In the first image, you can clearly see several peaks and valleys with Pure Cab disabled. However, with it set to just 0.1, those dips and peaks smooth out considerably. In a nutshell, that's what Pure Cab does, and in my opinion that's a good thing if your use case is recording, as the peaks and valleys often manifest as fizz and/or some other high end anomaly that isn't necessarily pleasing to the ear.
At its lowest setting Pure Cab does a good job of smoothing out and rounding off the high end in gainier profiles.