Just got home yesterday from a 7 day cruise. I use my Carnival Mastercard to buy everything and make weekly payments to avoid the interest charge. This past cruise I had enough points to get $600 off. It works if you know how to manage your money.
It can be used for other travel expenses but the benefit is best when used on a Carnival cruise. It more or less lets you get a rebate on your travel expenses.
All I ask is a tall ship and a star to steer her by; John Masefield
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
Celebrity: Summit 2011
Princess: Ruby 2010, 2014, Caribbean 2013, Coral 2014, Regal 2014, 2016, Royal 2017
Star Clippers: Royal Clipper 2015