This is useful for passing on any data to the listeners. By adding it to the options object when you set up the listener, you can access them easily. Also it allows you to do specific behavior in your listeners based on the options you used to set them up (like buffer, delay etc).
I noticed this because I have implemented a custom Ext.Direct provider for our Ext4Yii framework. The provider broke since the listener.o was not expected as an arguments item.