When someone such as a blogger is promoting a product (airline, hotel, credit card), it's in the interest of the reader to know if the blogger is being paid to promote it or if they're doing so of their own volition.

The credibility of the blooger is what matters. If they're being paid ($, flights, rooms, etc.) by Airline X, I'd like to know as it'll affect how much I believe their assessment of that airlines product. If they disclose that they’re compensated, I can then factor that into how much merit I put into their review. If they give glowing coverage to the new premier cabins on a flight to wherever, AND we know that same airline comped them the flight, they’re just a talking heads who’s opinion is worth little more than the slick ad’s in your inflight magazines.

Though it’s pretty easy to tell who’s shilling a product – just look for the never ending positive reviews, lack of any critical comments, and an overabundance of photos.