Using the Gain-Q interaction option, between the gain and Q knobs, you can enable a subtle, analog-EQ-like gain-Q interaction. When enabled, Q and gain influence each other in a pleasant way often found in analog mixing consoles. Essentially, this means that the Q automatically gets a bit narrower when gain is increased, and the other way around, a little gain is added when the Q gets very narrow. (From Fabfilter Pro-Q 2 manual)
The main motivation is to make it so there is there is less reason to use an analog style insert instead of the MixConsole EQ. Mixing using the knobs will be more like interacting with a classic console.