Context is king
If the Internet is the "information superhighway," maybe online marketing is the collection of billboards and roadsigns along the highway that encourage you to exit.
Gas, food, and lodging are the real-life staples because of context. It's what the users want and need. Online, context is the power behind Google Adwords and other similar programs. You pay for your ad to show up only when users are searching for a keyword related to your product or service. So the ad for your brand of ice cream doesn't show up when people are searching for "cars," but it does show up when people type in "desserts."
Similarly, promotions on your site can be more effective when placed "in context." People will be more receptive to an ad if it makes sense in their current surroundings.