It seems that most people here would like it very much if SEO died a horrible death as quickly as possible. Let me try and paint a (probably somewhat sketchy) picture of why it should exist (I have nothing to do with SEO consulting btw):
1. You have a business.
2. Said business has a website.
3. Said website is bringing in no discernable revenue for your business.
4. Poking around for reasons, you discover that your website ranks really lowly on Google.
5. You have no idea why.
6. You pay an SEO consultant. They take a look at your web site, and your overall web presence, to try and find out what's going wrong.
7. They discover that your markup is terrible, the wrong things are enclosed in header tags, etc. Also, you have little to no visibility outside of your website.
8. They advise some sensible changes to your markup, and force you to start posting things on Google+, Twitter, Facebook, etc.
9. Your ranking improves. As does your revenue.
Which bit of that process is evil and wrong?
Of course, there will always be a shady side to SEO (people taking it too far, etc), but there's a valid reason reason it exists: many people simply do not know enough to get the basic components right. SEO consultants help them do that, and improve revenue as a result.