While using only one ad network is fine for some projects, there are several problems that can arise. Using multiple ad networks can help resolve them. However, managing multiple ad networks can sometimes be a chore and require significant manpower to keep at optimum efficiency.
I'm not sure what this community's idiom for backing up answers is, but this answer is based on my experience in publisher-side ad operations, which includes optimizing ad network revenue. Although I have never worked with a mobile game, I have worked with mobile app advertising.
An ad network usually won't fill 100% of all the impressions your app requests. When that happens, the network can either leave the space blank or it may fill it with a free ad for a charitable cause or government program. For example, when AdSense doesn't have a paying ad available for mobile sites it will often provide an ad for the U.S. Forest Service ("only you can prevent forest fires").
You don't get much (if any) revenue for these, so its preferable if you can fill them with something else. Many publishers use multiple ad networks to resolve this. A common technique is daisy chaining: when your first ad network doesn't return a paid ad, you can ask a secondary ad network to fill it, and so on. Another (more difficult) approach is to allow each network to bid on the ad placement and return only the ad which is the most profitable. Some ad networks discourage this, so it's best to check into the terms of your agreement with them.
A second related, but less severe, problem is that sometimes an ad network will provide low-value ads. For example, your ad network may not have many advertisers who want to advertise on your particular app. One solution is to switch to an ad network which specializes in your audience, but that can be a pain - when their client base changes are you going to switch to a new ad network?
Instead, most publishers will use multiple ad networks to make sure that they are getting good deals. They will set a minimum CPM (cost per thousand) or CPC (cost per click) and only accept ads which meet those requirements. When their primary ad network doesn't return something suitable, ask the secondary network, and so on.
Ad networks only have a certain number of impressions to give away. If your app has a significant number of impressions in a small advertising market, you will quickly exhaust the high-value campaigns and your network will return lower-value campaigns.
Using multiple ad networks can help you maximize your revenue in this case. By opening yourself up to a larger pool of advertisers you will be less likely to exhaust the pool of higher-paid ads. In addition, you will be more likely to find advertisers interested in your particular app.