I've seen many references to medium improving the flow of your paint, but whenever I apply medium instead of water I notice no difference!

I've been painting since July 2016 and only recently discovered the use of glazing and NMM, which I'm trying to improve at. There have been many people telling me to mix lahmian medium in my paint instead of my usual water, and so I tried doing so. I've tried quite a few times now, and have noticed absolutely zero difference in paint consistency, flow and quality. Other than my paint mix getting a little bubblier. Anyone wanna explain exactly what it does?