Pareto Principle or 80/20 rule or Principle of Factor Sparsity:
"For many events, roughly 80% of the effects come from 20% of the causes."
The Pareto Principle, unfortunately, has been fully applied to software development usability. In fact, still today, 80% of the developed software is being wasted. An enourmous amount of money. We should develop software only after making sure it will be used.
"If the development team treats all requirements equally, a lot of code will be written to implement features the customer doesn't really care about." (The Art of lean Software Development)
"You gotta start with the customer experience and works backward to the technology. You can’t start with the technology and try to figure out where you are going to sell it." (Steve Jobs, WWDC 1997)
"There is nothing so useless as doing efficiently that which should not be done at all." - Peter Drucker.
"We now know that something between 85 and 90 percent of most software product features are unwanted and unneeded by customers. That is an enourmous ammount of waste of time and money that ends up on the floor." - Steve Blank.
"So, why does this happen? It’s simple: the person or group responsible for the productideas did not achieve a clear understanding of what the customer actually needed in order to solve their problems. (...) It’s clear that the way we build product is massively wasteful." - Build It Like A Startup, Greg Gehrich
The question is: How do we know which features users will use and which will not? User Driven Development is a methodology that helps answer this question early on in the development process, so you do not waste time and money on useless work.