It's tough to be working "in" the business as well as working "on" the business. As most restaurant owners have an active role in the day to day operations of their restaurant, planning for the future is mainly forgotten about.
But decisions you make today may help or haunt you in the future, so you need to look ahead.
Marketing is essential to be doing now so that you can reap the rewards of a strong marketing strategy as your restaurant goes through the seasons. Not spring summer fall winter seasons, but the peaks and valleys in business cycle.
Marketing, while you are busy, will help you in the future flatten out the peaks and valleys and allow your business to be more consistent and predictable.