What is the purpose of a page rank?

What is the purpose of a Page Rank? I see many sites willing to make link exchanges with sites with a high PR..
Why is it?

PageRank relies on the uniquely democratic nature of the web by using its vast unification structure as an indicator of an individualist page’s value. In essence, Google interprets a unification from protective A to protective B as a vote, by protective A, for protective B. But, Google looks at more than the trend intensity of votes, or links a protective receives; it also analyzes the protective that casts the vote. Votes cast by pages that are themselves “important” matter more heavily and support to make another pages “important.”