It was a response to the approach of carefully planning and specifying every aspect of the software up-front. The problem with that "waterfall" approach is tha

It was a response to the approach of carefully planning and specifying every aspect of the software up-front. The problem with that "waterfall" approach is that, in the real world, we often work in fast moving/evolving problem domains, or domains where we discover more as we build. Changing requirements are disastrous to the waterfall approach and can kill projects easily. If we're to be pragmatic at all, we need some flexibility/adaptability in the process.