The beauty about HELOC is the flexibility. You have it, you don't need to use it. When you need it, you start paying interest or principal, or you can pay only the interest without paying the principal during the term of the HELOC, but when the term ends which usually is 10 years, you have to pay everything back you owe.
When using a HELOC and paying the interest only as 老朽 has mentioned, I think that it is strategy to temporarily reduce your monthly payment to buy you some extra time to get your financing together.
For example, a $50K HELOC balance, with today's rate, you need to pay about $166/month for interest and $400/month for principal. With interest only payment, you will have $400/month to do something else for a short period of time.