Sales and Strategy Forecast: If you have a target to achieve you can plan out your work according. It's just like keep the figures real and amenable. Unrealistic figures would not help you ever to get the target on time. Secondly with these forecast, you can compare your marketing strategies and come up with the best solutions for future activities.
When I ask entrepreneurs and small business owners, it is not unusual for not having any marketing plan. I once read that Sir Richard Branson never had any marketing plan when he started the 1st Virgin business many years ago. In fact in one study of entrepreneurship (I couldn't recall the name of the study) was that as much as 50% of entrepreneurs and business owners don't have a business plan and marketing plan when they first started the business.