What changes about a Steam review when the user got it for free?

When you review a game, Steam gives you this option/checkbox:

But what does it change about your review? Does it change the review's effectiveness on the overall percent?

The only change, I know of, is that people that read your review see that, but I don't deem that information important and don't change my opinion about a game.

http://store.steampowered.com/news/24155/ states:

As of today, the recent and overall review scores we show at the top of a product page will no longer include reviews written by customers that activated the game through a Steam product key.

Customers that received the game from a source outside of Steam (e.g. via a giveaway site, purchased from another digital or retail store, or received for testing purposes from the developer) will still be able to write a review of the game on Steam to share their experience. These reviews will still be visible on the store page, but they will no longer contribute to the score.