Why change cooling fluid ?

Hello dear forum community, a few days ago we discussed in the association the necessity of changing cooling fluid controversially. Nobody knew exactly, so my feeling, for this the discussion was all the more excited. Really crazy, what moves people emotionally…. How is that really, if you only take into account the naked technical facts? 1. Is an exchange of cooling liquid at certain intervals necessary at all? 2. If yes, why only? Can’t water and will not get bad either !?