The most important aspect of business planning (the "What does it all come down to?" part), however, is spending the time to do your research ("due diligence") and critically thinking about these various aspects of your business. Of course, it is impossible to anticipate every detail simply by spending time thinking and writing; it is for this reason that plans are referred to and viewed as "dynamic". However, the more issues and scenarios you are able to come up with ahead of time, the more prepared you will be to handle these as they arise.
So, in the end, I encourage you to realign whatever preconceived notions you may have of the business planning process and view it not as a daunting task or a necessary evil of running a successful business. Instead, view it as yet another opportunity, the chance to help make your dreams into a reality by mapping out the needs of your business, your customers, your employees, your suppliers, your community...and yourself!
These areas require that you present a strong case for your proposed financial expectations, grounded firmly in the supporting information of your plan, including marketing, market analysis, business operations, and so forth. Having confidence in your business and in yourself will assist you in demonstrating the potential for your company and in being able to deliver what your investor is looking for. Doing your due diligence and knowing the facts surrounding your business and your market will prove to be of great benefit when selling your business case, both in writing and verbally, to the lender you are seeking funding from.
If your answer is no, you may have been able to easily accomplish this goal by planning for such expenses in advance through a business plan. Even if having cash at the ready is not a viable alternative for you, you may have planned to have a line of credit available for such opportunities, knowing that in your field these opportunities do arise from time to time.