Never book an overseas holiday without it. Especially when you are travelling to the US as they have one of the highest medical care costs in the world.
Buy it at the same time as you buy your flights. Why at the same time?
If you walk out of the travel agents or out of your home & you fall over & break your leg can’t travel, it means you are covered from that moment.
If when you get on the plane & arrive at your destination & the airline has lost your bags, you’re covered.

If you get to your hotel & it’s been burnt to the ground, you’re covered.

If you eat something bad & you get food positing & need to go to hospital, you’re covered.

Now if you didn’t have travel insurance you could be up for tens of thousands of dollars that you probably don’t have.
If you end up in hospital they won’t discharge you until the bill is paid. They will take your passport & you won’t be able to leave the country.
For the best policy for you do some research, get some recommendations & quotes from different companies. If you have a pre exciting condition it's best to call them.
Here are some of the things that has happened to our members, luckily for them they had travel insurance & were totally covered.
"Totally would not go without! My husband got Pneumonia when we were in Vegas 2 years ago and the bills came to a total of $110,000AUD! Crazy! (He is a type 1 diabetic and ended up being in hospital for 3 days) but goodness me. So thankful of we had insurance" Tash, Auckland, New Zealand
"I don’t know how people take such a gamble and travel without it. Two years ago while we were in US, the day before we were due to come home my mother in law had a nasty fall and ended up in emergency with broken ribs and a broken nose. Alliance were amazing and covered everything." Jane, Queensland, Australia