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"Why do we need it?" I guess you're talking about "Network Programming" and there are a lot of reasons why we need it. Here are a few examples I can think of: • Communication • Information sharing • Security (Physically and Virtually) • Analitics (Science and/or Data science) Communication has become one of the biggest Technology goals of today, we can nearly INSTANTLY communicate with anyone around the world! Why wouldn't we use this super power? We are using networks all around us, from WLAN (Wireless Local Area NETWORK) to house telephones that stretch all across the country, to airports who has a whole network system around the world to keep track of flights etc. We are using Network Programming for communication
I can't describe the all possible scenarios but you can get an idea by doing a simple google search. Last time i was using Java RMI where i was asked to develop a socket program that could connect to a specified port for communication.