Well you see there are many people who are all too concerned about being "better" then someone else to make theirselves feel better at the end of the day even though as you said its a virtual world that doesn't really matter. If you look further into those leaderboards and stats and all that you can see how much time these people actual have invested into playing the game to achieve that all so precious K/D ratio.
Other less concerned people just use it as a basis of how well someone can play. Obviously having someone with a ratio of .25 will not be too helpful to have on your team where as someone with a ratio of 4.00 will most likely be more effective and increase your chances of winning. Anything over 1 is usually good enough in my opinion as that means they at least get as much kills as deaths making them about the average player.