Let me use a quick example. Lets say you want to buy a new car. Typically you visit your local dealership. Over the past few months how many advertisements have you seen for cars like Volvo, Ford, Audi, etc? Are any of these cars not available in your city? Now imagine if you saw an advertisement for a Samsung car from South Korea. Clearly you can see the problem here and understand why this is completely useless. This scenario applies to every single continent and country. Most Ad Networks specialize in one continent, typically North America and Europe. To date I have not found one Ad Network which can truly optimize on a global level. Many claim global monetization, however you have to realize this is simply done by utilizing SSP / Exhanges to fill 100% of your inventory. This in turn ends up reducing your overall revenue. The key point to here. Find Ad Networks that specalize in a specifc region and optimize their strongest geos.
Passback tags, sometimes refered to as default tags, allow unfilled inventory to be sent to another Ad Network. Setting floor rates is another benefit of using passback tags. If you do not set a floor rate then you allow your inventory to be sold at very low rates. I have personally seen companies pay $0.01 eCPM which is likely less than the cost of operating the Ad Server. By setting a floor rate of $0.09 eCPM you protect yourself from two things. First, losing revenue to advertisers hunting for a bargin. Secondly, anyone advertising under $0.09 likely has a horrible product, service, or offer. The quality of the Ad Network itself might be worth questioning. Otherwise it might be worth noting that the floor rate might be $0.99 eCPM for some websites while others are $0.14 eCPM. Regardless it is an important step in optimizing your revenue.