Why should I optimize for Search Engines?

Over 4 Billion dollars on search engine marketing was spent according to SEMPO (Search Engine Marketing Professional Organization)

87% of people who use the internet find web sites through search engines. (Georgia Tech)
43% of advertisers respondents have changed their budgets from other marketing techniques to organic search engine results. (SEMPO)
92% of online consumers use search engines to research and buy online (NPD Group)
Direct Marketing Association determined that search engine positioning was the top method of driving traffic to their sites. 66% search engine positioning, and second was email marketing with 54%.
55% of online purchases came from the search engines as opposed to banners which was only 9%. (NPD group)
Over $95.7 billion dollars were spent online in 2003! And they estimate the total to come to $229 billion dollars by 2008! (Forrester Research)
Internet users are 20 times more likely to click on search engine listings instead of banners or tiles. (NPD Group)
According to a recent Jupiter Research Survey, searching on the search engines is one of the main uses of the internet among 79% of users.
Stats of how internet users find web sites according to Georgia Tech:
87% Search Engines

85% Links from Other Sites

63% Printed Media

58% Word of Mouth

32% Newsgroups

28% Books
Iprospect did a survey in 2002 that reported that 78% of web users abandon their search if the first three pages didn't show what they were looking for, and 28% didn't scroll past the 2nd page of results.
Search Engine Rankings in January 2004 according to Nielsen/NetRatings:
39.4% Google

30.4% Yahoo!

29.6% MSN