Eric Ries explains why a having a great plan and executing well worth nothing if your company is far away from the market realities:

I had the privilege, and the misfortune, to be involved with a startup that executed this plan flawlessly. It took years, tens of millions of dollars, and the efforts of hundreds of talented people to pull it off. [...] Only this company was a colossal failure. It never generated positive returns for its investors, and most of its employees walked away dejected [...] This company was shackled by shadow beliefs that turned all those good intentions, and all that successful execution, into a huge pile of wasted effort.