According to me if you will subscribe for Google ad sense they your subscription will be denied without traffic. If someone is going to search a keyword in Google then it is must to optimize it without optimization it will not be displayed in top 3 pages.
Hmm. That's a great question Why page rank is important? have you observed when you search in Google or yahoo you get many websites in results. You might be having a website and you are not doing its Optimization is search engine so it has no page rank and hence it will not be displayed in top 100 results or may be in top 300 results.