Imagine if you can do all these and manage all these in a marketing plan? Systematic, organized, well thought, innovative and captured all your pertinent business information, don't you find having a marketing plan is useful for your business?
Risk _ in our marketing plan we shouldn't only concern about making money or positive points, we also should address the risk we are taking. for example it is common for a marketing plan to capture things like, "require additional staff to handle sales call", "additional investment from the management budget will increase chances of increasing 10% sales target" and many more. Imagine if you can factor your risk earlier and think through it, wouldn't it be better for you?