Google AdWords offers excellent controls for advertising to ensure that you get the most value for your money. One way to do that is with setting times under ad scheduling. Google AdWords enables you to set the time for when you want ads to run. For many people, this might not seem very important, but it can be actually very useful.
Restaurants have an obvious use for this. Timing would allow them to run one ad for lunch and a different ad for dinner. Maybe they have excellent lunch specials but are much classier and more expensive for dinner. They could run one set of ads from 10 AM until 2 PM, promoting this lunch specials as people look for some place to have lunch. Then, later in the day, perhaps around 5 or 6 PM, they could have another type of ad promoting their dinner services. With that, they would not need to worry about lunch diners seeing dinner prices and staying away or dinner guests getting the wrong impression from seeing their lunch specials.
Another example might be a company doing business-to-business sales, lets say for a customer relationship management system (CRM). It would make sense that most of their sales come during the business day. However, there may be plenty of people who also do similar searches during the night but are typically small business people or solo entrepreneurs looking for a less expensive solution. If you want to avoid wasting money advertising to them, you could just set the ads to run during the day and not during the night.
Those are just a few cases where you might want to maximize the effectiveness of your advertising by controlling the timing of the ads. For more tips on running AdWords campaigns, check out AdWords University: The Complete Guide to AdWords.