When I first heard the term ‘sunk costs’ I had no idea what it meant. However, I heard it from Seth Godin so I payed attention and came to find that it referred to money that had already been spent (which you can’t get back) that should not be considered in future decision making.
This idea can be applied to more than just money, it can also be applied to time although they say time is money so it all really just boils down to the same thing.
The idea of sunk costs is so useful because often we make decisions based on the past whereas sunk costs advises us to put that aside and make choices based on the present. I think it helps us to become more rational decision makers instead of relying on how things of the past have made us feel.