Most transactions have an upside and downside but when something is free, we forget the downside. “Free” gives us an emotional charge that we rate what is being offered more valuable than it really is.
Why? Because people are afraid of loss. The real power of “free” is tied to this fear. There is no VISIBLE possibility of loss when something is free. People rather take the free Tesco chocolate than paying 10 penny for Lindt. With Lindt, there is the possibility of disappointment - so we chose the risk free, “free” option.