The area in which the keyword is found plays a key role for many searches. Having the keyword in the Title tag on most search engines will give more relevance to the page than the same keyword appearing in the body area. To rank well generally requires you have keywords in many of the areas in which a search engine looks.
The areas that are the most important will vary by search engine. Examples of "areas" of a page are Title, Heading, Link Text, and Body. See Areas for more information.