@bythewell - It's important in all areas of life really. I think often businesses fail because the people involved didn't pay enough attention to detail.
After all, it's one thing to say that you're going to run a business where you sell cookies. It's another to look at the details of who you are marketing to, what makes your product different and special, what your mark up is going to be and whether that's viable and so forth.
I think all too often people get lost in the grand picture of "we're going to sell a lot of cookies!" and ignore the details of "well, it costs more to make them than we are getting in returns".