[MikkelRS]

The fluid itself has parts that break down over time and mileage, this is why they suggest changing it so often. In the real world, if you're not hard on it and drive it fairly regularly and keep an eye on how it shifts, then just check the fluid every so often and change every 100k or so, or change it when it starts shifting a little different.  

The wet clutch packs and bands inside the transmission rely on the fluid to have certain 'friction modifiers' in it to have the proper slip-to-grab ratio of the components. These break down over time or hard useage. This is why our transmissions require mercon V only, it has a formulation for this.