Thanks for mentioning our software!
AcouFiend is indeed capable of simulating feedback, but also sustain (use harmonic #1), e-bow tones and more. The idea of such a simulator as a plug-in is to either generate feedback/sustain at low volume while playing (for example on stage if you are using in-ear monitors and no amp, which happens more and more these days, or at home if you don't want to disturb neighbors), or to add feedback to existing tracks, after they have been recorded (it works better if you have the original dry signal of the guitar and re-amp it).
It also works on non-guitar signals, as shown in the lead synth example below:
It can create some pretty weird effects too, when used in the full wet path of a reverb or delay.