Marketing needs to show results — new contacts, more engagement, fresh opportunities to do more business. It's great to 'think big', but not if it leads to delay. The world won't wait for you to put your biggest ideas into action — it's moving on and changing all the time.
An Agile approach encourages us to iterate, to pick off the 'low hanging fruit' first and test new ideas in their most basic form first. It's efficient, builds up invaluable feedback — and we can add the frills later.