It can improve the audio by peaking the frequency you desire. You can use it as a highpass of lowpass filter. This type of filter is good on CW contacts or with a bit of tweaking, RTTY. On phone operation the improvement is sometimes difficult to hear.
In it's day the Autek qf 1 was really good. Today you can get audio DSP units that do much better but cost an awful lot more. You can get them used and pay less.
Audio conditioning is often not overly impressive since the interfering signals are already amplified and fed to all the circuits of the rig. Still it's better than nothing.
Well I recieved the qf1a and I'm impressed. I some cases it will allow you to make a contact on cw, where you could not with out it.I use it in conjunction with my cw filter. If conditions are not that bad, it still amplifies and allows you to taylor the tone to your liking. It's well worth the $50 they are going for on ebay.