Call it Agile, Lean Startups or any other name — what we are trying is to,

1. Get as close to the customer as you can — even include them in the development and ideation processes if possible to hear their ‘voices’ early [Voice of the customer]
2. Churn out working prototypes ASAP — you do not want to wait for 6 months to find out you have been building it wrong [Fail and course correct faster]
3. Make the best use of your resources — How can i achieve more with less? This calls for discipline and a change in mindset that all these approaches/frameworks espouse [Less is better]

I use a combination of these methods — even our good old SDLC where required to deliver the best, every single time.