OPC is just a standard to provide O
roductivity and C
onnectivity. It is used in many data systems, e.g. process control and provide a controlled interface to access data/system.
Tere is not alot to say about why, or what, or when, its totally dependant on the system integrators on how an why they choose this technology over anything else.
The main benefits are obviously that it provides an open standard that manufacturers and integrators to work with, and thus extend the reach and integration capabilities of their products.
Read more about it at the http://www.opcfoundation.org/