The length of the solar cycle is not always 11 years. The shortest
cycle on record was 7 years and the longest was 14 years. You can also see
in the sunspot number graph that the maximum
number of sunspots is not always the same.
Although sunspots don't have any effect on us at the Earth, solar activity, like flares and coronal mass ejections can affect us on Earth. We can use the number of sunspots as a measure of the level of solar activity because when this peaks so do all the other kinds of solar activity, and they can affect us.
By predicting how many sunspots there will be, we can tell roughly how much solar activity to expect.
How do we predict the number of spots?
Perhaps the best way to explain this, is to let you have a go at doing
it. The basic idea is to look at the last few sunspot numbers and try and
get a feel for what the next one's going to be.