A highly-rated television show may be seen by 10 million viewers. Imagine transmitting that content on the Internet using unicast. A high definition stream of reasonable quality can be transmitted at 10Mbps. Transmitting 10 million of those 10Mbps streams on the Internet would require 100Tbps of bandwidth and a video server capable of originating that many streams.
Transmitting this much content simply cannot be done economically on the Internet using unicast. Content delivery networks (CDN), which are used to distribute the content load across the network geographically, could not appreciably change this equation. CDNs would merely distribute the problem. On the unicast-only Internet, the cost of transmitting simultaneous streams increases linearly with the size of the audience.
However with multicast, a video server with only the capacity and bandwidth of a single 10Mbps stream could deliver that same show to all 10 million viewers. Armed only with the Internet connectivity commonly available to broadband subscribers, one could easily transmit a single high-definition video stream that could be simultaneously viewed by much of humanity. This is because the cost of transmitting remains constant regardless of the size of the audience. Whether there is one viewer, or 1 billion, the cost of transmission is the same for the source.
AMT does introduce some per-viewer costs to the network provider since it does replicate to the unicast world. However, this is a fraction of the cost of typical unicast delivery, even with CDNs, as the replication point (the AMT relay) can be built into the network infrastructure and placed at the edge of the multicast-enabled world. Also, as AMT succeeds, native multicast becomes an increasingly attractive option for network providers. AMT is really just a (necessary) interim step toward full (or near-full) multicast connectivity on the Internet.
Now it is fair to point out that our sample television show with 10 million viewers wouldn't necessarily have 10 million simultaneous viewers. TV shows are typically staggered across the different time zones. Also, DVRs have made it fairly common for viewers to record shows to watch at a later time. However, it should also be pointed out that there is much television content, like sporting events, that is typically consumed live. For example, 106 million viewers worldwide tuned in to watch the Saints defeat the Colts in Super Bowl XLIV. The vast majority watched this game on live TV.