Without that, the car effectively has to guess how much capacity the batteries has available. It does this by keeping track of charges & voltage, but it can only be so accurate. Imagine filling a cup 66% of the way full of water every day and draining some of it again. Every day some unknown amount of dirt accumulates and stays in the bottom of the cup. You can measure how much water you're letting in and letting out, and you can weigh the cup, but you can't measure how much dirt has accumulated. How accurate will you be in hitting that 66% mark after 2 years? Not very. HOWEVER, if I allow you to fill the cup all the way and drain it once - voila, you immediately know where the 66% mark is now and you can start filling / draining it correctly again until the dirt factor becomes significant again.