The benefit is if you need to cancel your trip the points go back in your acct and you can have the money for the taxes/fee refunded to your credit card as long as you check the box that says to refund that money to you. Points can also be used to book flights for anyone and if the other person cancels you get your points back.
The minimum you can buy is 2000 points for $55. Ever once and awhile they run a special and you can get 30% to 40% bonus points with your purchase.