Article Marketers: Let's discuss why we spin our articles

Fellow internet marketers,

I've been involved in article marketing for several months now (I know, I'm a newb) but I want to set something straight. We all know that we need to spin our articles, and we all do it, but does anyone actually know the reasons WHY? Spinning articles is a big time commitment. It usually takes me at least an hour to spin a 500-word article 3 times at the sentence level. So why do we do it?

Here's what I've been able to figure out so far, and I invite you correct me if I'm wrong. Why spin our articles so that:

1.) More directories will accept the content since it's more likely to pass in-house duplicate content filters.

2.) Google is more likely to index the article, thus earning you a backlink (If you submit a spun article to 100 different directories and Google decides that 90 of them are duplicate content, it won't index those 90 articles, so you don't get the backlinks from them... right?)

That's what I've been able to figure out so far. What am I missing? Why else do we spend so much time spinning our articles? I am to the point now where I am outsourcing article writing and spinning, but I find it a little funny that I'm not even 100% sure I know why.