Google has been trying for a long time to bypass the carriers with its Nexus devices, but unfortunately the Nexus devices hasn't been very popular, especially in the early days, because neither Google themselves nor the OEM's making them were advertising them. Plus, it was very hard to support more than one carrier at a time, since Google had to make special deals with them.
The Nexus phones have become a little more popular lately, the $350 price from the Play Store, or the $450 unlocked price from carriers and retailers, may still seem quite a lot of money for most people, when they could just buy a phone for "$200 or less" on contract.
That's where the Moto G comes in, because it starts at even less than $200, off-contract, making it the perfect phone to buy with your T-mobile contract, or as a prepay devices, or simply as a good enough unlocked device. The Moto X and the Nexus 5 remain the higher end phones, but for most people the Moto G is exactly what they need. It has a quad core processor (not the fastest, but good enough), an HD screen that's sharper than any other mid-range phone out there, and a great 5MP camera.
The reason why the Moto G could be more important to Google than either the Moto X and Nexus 5 is because first off, the Moto X wasn't that featured packed to begin with, for the high-end, and it was mainly sold in US. Second, the Nexus 5 may do okay in US, but in most other countries where people buy cheap unlocked phones or free ones on low-cost contracts, the Nexus 5 remains prohibitively expensive (~$600 in countries where you can't buy it from the Play Store).
So the Moto G is a much more global and a more flexible device for all sorts of people, from those who are really sensitive about price in the richer countries, to most people in non-US markets, who don't want to pay a whole lot for their device, but want a decent phone. With the Moto G, and its successors, powered by KitKat or better, Google could reach hundreds of millions of customers in the future, all by themselves.