The site explains itself as such:
Why it's useful?
See if your potential game purchase is worth your hard earned money.
Find out just how long that backlog will take to complete.
Estimate how much longer your current game will last.
Compare your game times to other players.
How it works?
Check the column that best matches your play style, this is the average time it will take you to complete the game. You can also click on the games individually to really break down the stats.
Main Story (Required) - Gamer completed the main objectives, just enough to see the credits roll.
Main Story and addition quests/medals/unlockables - This gamer likes to take his time, discover and complete additional tasks not required.
Completionist (100%) - Gamers that get every achivement, every medal and conquer all that the game has to offer.