The chances are slim that a person living in poverty in a developing nation has access to the Internet on a computer. It's expensive and, in some places, there's a lack of infrastructure to support it.
The chances are better, though, that that person owns a cellphone. It's probably not an iPhone or an Android, and he or she probably hasn't purchased a data plan for it, but it has the ability to access the Internet.
Google believes that this category of cellphone user is the future of its expansion.