Twenty / Eighty is a pervasive concept. It's both about time to market and solution for the masses..

If you need to accomplish quickly some task, you'll need to focus on the core of the problem. For example, usually with the 20% of the "code" you get an 80% finished running prototype, and you'll be able to give a new service to a customer in a hurry (today, time to market is key to stay alive)