why do spiders have 8 eyes?

[Niobium_Knight]

and how do they manage to use 8 eyes at once? does the vision join up totally into 360 degrees or do they only concentrate on a certain area at any one time?

[bibliophage]

I searched http://www.google.com for spiders “eight eyes” and the first hit was this one

. Most spiders aren’t known for their vision.
. . .
But a few spiders, most notably jumpers, have excellent vision. Jumping spiders have two big eyes in the center of their faces. These eyes are similar to ours. . . . The other six eyes, which are smaller and arranged in rows on the face and upper body of the spider, are used as motion detectors.
link

[coffeecat]

The better to see you with, my dear.