Traditional marketing mediums like television, print advertising, billboards, etc. provide very little in terms of data to measure your return on investment.
Though 10,000 cars might drive by a sign each day, how many pay attention? How many care? And even if it results in a new customer, how is that measured? Just because a tv or radio station broadcasts to 3 million residents, how many are listening at the exact moment your ad is on?
With most online marketing, you can see how many people click through, how long they spend on your site, and see what your sales conversion rate is. You only pay for the customers that take act (click through), not the ones who happen to be in the room. Campaigns are also much less expensive.
That being said, look at the list of the fastest growing companies in the country. How many of these companies have you seen advertising to a general audience on television, in movie theaters, or in the newspaper? Hardly any. How many have been leaner in their efforts by focusing on niche audiences and specific industries through targeted marketing (especially online)? All of them.
Food for thought.