Pitchbend Adder is a simple JSFX script used to add an audio signal to Pitchbend events on the channels specified. The audio signal should be "normalized" between -1 and 1, representing the pitchbend range, which gets added to the pitchbend messages already present in the MIDI bus (but also creating new pitchbend messages if needed). So be careful! You can actually get out of range quite easily (it will get capped to max/min values in such a case). Other MIDI events are passed through. The input audio signal that gets added to the Pitchbend is mono, and it is added to all the channels specified. If you use it in a stereo configuration, it will probably be the left channel, so keep it in mind.
As you can imagine, the purpose of this little script is to be able to manipulate Pitchbend from audio, while adding it on top of other Pitchbend messages already present. If there are no such messages, then it will simply create Pitchbend messages out of the audio signal (because it is added to zero), while its precision is not limited to buffer chunks (unlike ReaControlMIDI), so it is superior even if you use it only to create pitchbends out of audio signals.