Two major considerations differentiate marketing strategies from one another.
The first consideration is that the bottom line is what really matters. Having more people interested in your brand and talking about it is nice, but hearing someone mention your name on Twitter doesn't usually equate to making a sale.
For most marketers, the bottom line is what really counts. What's going to bring you in the most money?
The second consideration is that "good enough" isn't good enough; you want the best.
There are tons of marketing strategies out there, each with different possible returns and different demands. Many of them offer promising returns on your investment. But if you're going to pay X amount of money for a service, you don't want just a "positive" return—you want the greatest possible positive return.
With that in mind, you must find out which marketing strategy is the best one for you to use.
Objective metrics are hard to cite, however. Every business is going to see different results, and there's always the possibility that some of the businesses in our samples won't be using the strategies in question correctly. But there is one mathematical reason why SEO, when done properly, fulfills the above condition.