The majority of Internet traffic uses unicast data delivery. In unicast, a server transmits data directly to the client requesting it. Each client requesting data gets its own stream from the server. The cost of unicast delivery increases linearly with the audience size, as the source must be powerful enough to transmit a duplicate stream to every interested end device, and the links on the network must have enough bandwidth to handle all the duplicate streams.
By contrast, broadcast data delivery allows a server to send a single stream to the network, which will be received by all end users (whether they are interested in the data or not). For example, an old-fashioned over-the-air radio station broadcasts its signal to all radios within a given area. If a radio station were to use unicast delivery, it would transmit a separate signal to each interested listener. If there were 100 interested listeners, a "unicast" radio station would have to transmit 100 different signals. Instead, a radio station broadcasts a single signal that all radios receive.
The benefit of broadcasting traffic is most obvious for the owner of content - whether there is one listener or 1 million listeners, the cost to transmit remains the same. The disadvantage is that the traffic is sent to all users, whether they are interested or not. By comparison, unicast traffic is delivered only to users that explicitly request it. Unicast traffic will not be flooded to uninterested users.
Broadcast works well in limited geographic areas or on small networks. However, on the Internet, which connects millions of networks and billions of end devices, broadcasting traffic to all those end devices is simply not feasible. Hence, unicast is much better suited to the Internet, even if it is inefficient in delivering multi-destination traffic.
Between the two extremes of unicast and broadcast stands a third option: multicast. In multicast, the source transmits a single stream. However, the network intelligently determines where that content is desired and delivers the stream only to interested receivers. By delivering a single stream of data, a multicast source enjoys the same efficiency of broadcast in that the cost of transmission remains constant whether the audience consists of one person or a million people. And by delivering that content only to end users who are actually interested in the content, multicast enjoys a similar efficiency of unicast in that traffic is not flooded to end users who have no interest in the content. Multicast enjoys a "best of both worlds" by realizing the benefits of unicast and broadcast without suffering their deficiencies.