Clients of your business shouldn't need to see your business plan. If they are worried that you will be in business next month, you could reassure them in other ways. You could tell them about other clients that you have, or other new business that is coming in.
On the other hand, a potential investor in your business will absolutely need to see your business plan. Afterall, they are risking their money on your business. It's only right that they should see your future plans, your sales forecast, etc. They will want to know how you plan to use their money and what kind of return on investment they are going to see if you are successful. An investor that doesn't ask to see a business plan is not usually an investor that you should do business with.
Good luck with your business!