Oahu in September and October gives you warm water, fewer crowds on the North Shore, and the best flight deals of the year. You spend your days surfing at Waikiki, hiking Diamond Head, and eating garlic shrimp from food trucks without waiting an hour in line.
When you are looking at the typical $3200 price tag for the whole trip, paying it all at once can sting. Spreading the cost out makes it manageable if you plan it right from the start.
The 0% intro period is a deadline, not free money. If you still have a balance left when those 18 months are up, the remaining amount will start racking up interest fast.