Before we figure out why spiders have so many understand we should first ask; why do humans have two eyes? The answer is because we have evolved to need them. Humans use two eyes to be able to see three-dimensional objects, to judge how far away they are, how tall they are and how wide. Spiders have developed eight eyes (usually) for the same reason; because they need them.
Spiders are small predators and have evolved based on their needs over time. The majority of spiders have eight eyes although some have six and some none at all. It all depends on the environment they evolved in and what they needed them for. The most common spiders have two large eyes in the center of their head. These eyes perform much the same role as human eyes. They are there to judge distance and to see prey clearly. However, most spiders also have four more eyes on the side of their head. This is because spiders have no neck and so they use extra eyes to watch out for predators and prey.
If you consider the spider as an arachnid that lives in a circular web you can start to understand why they always want a wide field of vision. They need to be able to watch the whole web while they sit in the middle. This enables them to see any looming predator approaching. Jumping spiders similar use their side-eyes to spot any notable movement and they can quickly leap into the air to catch an insect.
Some spiders have no eyes at all and this just goes to prove that evolution is the cause for all of these differences. Some spiders that dwell in dark caves have no use for eyes. Over time evolution stopped wasting energy producing eyes that could not be used properly and allowed these cave-dwelling spiders to develop their other senses.
Who knows what the future holds for our species. Perhaps as our generations continue to stare down at mobile devices we will develop an extra eye on our chin so that we can watch our phones while crossing the street. While an evolution like this would take many thousands of years to take place, we could probably use it sooner.
Let us know your thoughts on the impacts of evolution and any interesting animal quirks you have found as a result of evolution.