I know that JSON is another way for data transmission like XML. Now there might be some data transmitted with JSON tags (like in xml tags). so my questions is why people are using JSON. secondly how would we know that the server is returning JSON ...?
JSON is a lightweight format, while XML is a considered a heavy weight format. So, to transmit the same amount data, you'll need fewer bytes in JSON,which may improve the performance if you are sending lots of data.
It is also considered more "human readable" than XML.
As for how you know if a server is sending JSON data, it should say so in the mime type in the headers.
You can figure out that your server returns JSON by i.e. looking into "Content-Type" HTTP Response Header, if it contains
application/json - the server is talking JSON.
In regards to JMeter and JSON - in 99% of cases it would be REST API load testing, to learn how it could be done using JMeter - check out Testing SOAP/REST Web Services Using JMeter guide.