The Simple English Wikipedia was made because English learners and English teachers need it. The pages there are more easy to read, and people can read them in school, use them for computer work, etc. Non-English Wikipedias can also translate from the articles here.
What is Simple English Wikipedia?
Simple English uses fewer words and easier grammar than the English Wikipedia. This is not its only difference. It is for different people with different needs: students, children, and adults with learning problems.
When the Simple English Wikipedia began, the original English Wikipedia already had 150,000 articles. Seven other Wikipedias in other languages had over 15,000. So most Simple English articles take old articles and make them simple; they are not often new articles. Because of this, Simple English articles can be a good way to understand complicated English ones: if someone cannot understand an idea in complicated English they can read the Simple article. So it is quite important to have "interwiki links" to and from the other Wikipedias. Also, it is good to constantly look at all versions in all languages, to get new ideas. Wikipedia needs this to make it better.
For more information, see this page on Simple English Wikipedia: Simple English Wikipedia.