This is used to lower the surface tension of the paint or fuild so it can more easily 'flow' into the recesses of the models. This is great for thinning down washes and inks. It helps the ink go into the crevices and limits pooling that can occur with a thick wash to just using water to thin it. If you gloss coat your model first it will flow even easier.
Another thing you can do is make you own wash out of regular paints. Here I mixed up a wash of my base color to give the model more depth. It was approximately a 10:1 ratio, but all paints have different properties so you'll have to adjust as you see fit.
I used vallejo terran earth to make a wash onto a bone colored model. I used a brush with long bristles and a sharp point to hold more of the mixture and control where I wanted it. I am deliberate where I wanted the wash, not going over the entire model. That will make it act more like a glaze, tinting the underlying color.
Here is a finished example of the washing technique. Clean up is easy since I can go back over the bone color without having the trouble of a more liberal wash tinting the top color.
The flow aid has a ton of uses for thinning washes and glazes and making your own out of basic paint. Since this helps reduce surface tension to run into recesses, I wouldn't use it to thin paint for airbrush. For all the utility of the this product and the price of a 4oz. bottle, I give this product a top rating. 5 out of 5 shields.