Here's an important truth about advertising: nobody in advertising really knows if a tactic is going to work until it's tested.

In the advertising world, David Ogilvy is a legend. He built, from nothing, one of the world's most powerful and successful advertising agencies: Ogilvy and Mather. With a portion of the money he made from the advertising business, he bought a Chateau in France: Touffou.

Ogilvy had strong feelings about advertising. Here's one of them.

"The most important word in the vocabulary of advertising is TEST. If you test your advertising, you will do well in the marketplace."

Here's an important truth about advertising: nobody in advertising really knows if a tactic is going to work until it's tested. You can hire a group of advertising experts with decades of experience but they won't really know if a campaign or website launch has worked until they get the data back from the real experts: the potential customers.

This presents good news and bad news. The bad news: you can spend a lot of money quickly and get little or no ROI. The good news: you can quickly make changes and constantly tweak campaigns so ROI steadily improves.

It's tempting to look at hiring a digital advertising agency as 'a must' purely because it's important to have a presence in the digital environment. "Everyone else is online so I must be online."

No. The most important reason to hire a digital advertising agency is to maximize the impact and ROI of every penny you spend on marketing.

Properly built, a digital marketing platform lets a company precisely determine where and how to spend money—in order to make more money. The foundation of the digital platform is testing. An experienced digital marketing agency can help you produce the creative elements (like a website, email campaign, or landing page) then will keep testing the elements to improve ROI.

Let's say you decide to start advertising through Google AdWords. Many companies make the dire mistake of using Google's instructions, coupled with their own intuition, to run their AdWords spend. Perhaps the person in charge has read a book or a blog about AdWords. Very quickly, you can pay what digital marketing insiders call: "Google's Stupid Tax." This means you're giving thousands (perhaps more) to Google and getting nothing in return by not following the best practices—and these best practices change weekly.

So one benefit of partnering with a digital marketing company is having people on your side who can maximize the effectiveness of your pay per click advertising, keep you from wasting money, and help you earn a potentially significant ROI.

It's a little known secret: some companies have an unlimited digital advertising budget.

These companies, using the latest testing and web analytics, find a 'sweet spot' in their online advertising that earns a certain return and profit. Once they find this sweet spot, they pound money into that sweet spot—until it's no longer profitable.

An online advertising agency can help you find that sweet spot.

Companies hire digital marketing agencies for additional reasons.

• No need to find and hire full-time digital marketing employees.
• Access to top creative talent to design and build a digital platform.
• You can find clarity in a sometimes foggy world.
• There's no need to invest in digital marketing tools—some of which can be expensive.
• You don't know anything about digital marketing and the complexity is somewhat overwhelming.
• You want to test and experiment to see if online marketing will provide an ROI for your company.
• You've been burned by advertising agencies who win prizes, send large bills, but never show tangible results.
• You're interested in tangible ROI!

The promise of digital marketing is tremendous: it can transform a business by precisely managing every penny of a company's marketing budget. But the key is testing. Again from David Ogilvy:

"Never stop testing, and your advertising will never stop improving."