The executive summary is basically your preface to your report. It is a critical part of your business plan or investment proposal which enables your readers to know what you are proposing to do and why she or he should spend more of their precious time on reading your in-depth proposal or report. Basically, it is the document that advertises your intent and will grab their attention and get them interested in your proposal.
While considering and analyzing whether to fund your project at the initial stages of the project, the executive summary may be the only document that the investors read. So it must capture the essence of your business. By discarding unnecessary details (that are already covered in your in-depth report) and focusing on the essential part of your story, you get a better idea of what your plan is and how to sell it.
Your target audience (lenders, investors, CEOs, executives or managers) are busy people. Time is of the essence. If you start by handing over a thick manuscript with all in-depth analysis of your proposal, more often than not it will stay at their desk unread for eons (if at all you convince them to take it).