A site on Mother's Day could sell a huge range of gifts for mothers. You can also increase the click-through rate by sandwiching a paragraph describing a product between two similar banner ads. Some companies encourage you to do that but some won't even allow you to write a few words beside their banner ad. Read the contract carefully. If you have managed to find advertisers, you're probably like most webmasters and don't have enough of them.
Associate programs can be a convenient way of filling the unsold gaps.
Here's another reason why associate programs are great. Because many of the associate programs allow you to use links instead of banner ads, you can have faster loading pages. That link is to my favourite associate program - favourite because you don't have to persuade people to buy anything. All they have to do is ask for a free sample of a Web Card, a postcard with a picture of your site on it. It's as simple as that.