A 0% purchase card allows you to spend on your credit card without being charged interest for the 0% period.
This means that if you wanted to spread the cost of a big purchase or a number of small purchases, you could make the payments on your credit card, then pay it off in monthly instalments before the end of your 0% period.
Just remember that once the 0% period ends, the card’s usual interest rate will apply to any remaining balance.