Instructions come with the product, but the couple I've seen say to remove the filters, and add the product to the water, a day or so before changing it, or to run the pumps for a minimum length of time, etc...
With the air shut off.
I'll leave it for you to decide if you need the stuff or not.
Some who've posted about it have been amazed at what their water looked like.
Personally, I flush once per year. I figure it's not all the expensive, and it can't hurt.
Sure makes an impressive stack of bubbles!