Do you remember some post-mortem meetings you have been attending? Usually, they just take place when something went really wrong and then they often turn out to be an emotional blame game.
The idea as such is great and very important for the development of individuals, teams and even organizations. That's why retrospectives - in short "retros" - came up. It started in the engineering and software environment. Nowadays, it is used in all different departments, teams, organizations, industries and countries.