I haven't used it myself, but at a glance it seems like an action that's best suited for debugging, or general tracking of what states are firing where and when.
If you understand the basic premise of how State Machines work in PlayMaker, this action will output each of the requested datatype. So for example if you have an event that sets a float somewhere, you can use this action to determine the value of that float, what object and FSM it came from, and so on.
I would probably use it for debugging my scripts, but that's about it. I could see it being useful for some level of automated reporting and assembling of data for the purpose of feedback in a more complex State Machine as well.
Take it with a grain of salt, because I really just read the description and poked around with it in order to answer your question. Take it with a grain of salt.