Compaired to broadcasts and Multicasts, a Unicast is very simple and one of the most common data transmissions in a network. This article explains what unicats are and how they are used within the Local Area Network to allow hosts to communication with each other.
The Reason for Unicast
Well it's pretty obvious why they came up with Unicasts, imagine trying to send data between 2 computers on a network, using broadcasts ! All you would get would be a very slow transfer and possibly a conjested network with low bandwidth availability.
Data transfers are almost all of the times, unicasts. You have the sender e.g a webserver and the receiver e.g a workstation. Data is transfered between these two hosts only, where as a broadcast or a multicast is destined either everyone or just a group of computers.