Well think about this, if i wanna but some potatoes and only u have it then i will have to come to you! And if u want the money u have to crawl around to sell ur potatoes so that u can earn money!
Many factors are involved in SEO! Take this if you have just 100MB B/W then u might not have more than 1000 visitors per day! But if this crosses it would be like updation of your database and website! This is where Crawler's net comes in!