Most clocks use UTC time, which has leap seconds, designed to correct for the fact that the Earth is very inaccurate in its rotation.
I'm not sure if I'm correct, but what I've heard was that leap seconds have nothing to do with inaccuracy. Leap seconds happen because the Earth's rotation is slowing down (every day is a little longer than the previous one, about 60 nanoseconds shorter). This slowing down happens because the Moon causes a lot of water to move around the planet every day (known as the tides), which causes a lot of friction. This will lower the angular momentum of the earth slowly, causing days to become slightly longer.
So the days are now on average a little longer than 24*60*60 seconds, which means that we have to add a second every couple of years. Since days become longer and longer (but of course this will be in the order of milliseconds) we have to have leap seconds more frequently in the future.