It allows you to pick various attack and defense cards and run battle simulations to predict win/loss percentages. It also shows you the total boosts/degrades for various cards.
For anyone working on their decks, or wanting to see how they hold up against a Captain America defense wall, it's a great tool. I've seen a few others like this, but this one is much easier to use and much easier to read.
Pretty sure it works based on number of victories. Three victories in a row and you get 30%. Though, the Battle page of the wiki seems to indicate otherwise, and I honestly haven't paid that much attention lol.
I think it's important to note that it's a simulator. I think you should also look at what it marks as your highest potential Attack with the addition of the combo. I know that my own usually makes up around 5-7k on average depending on prior ability boosts, which can potentially make up the difference in power loss for substituting one stronger card for one weaker.
This obviously depends a lot on the cards you're choosing though.