why we use cascade connection

     what is use of cascade connection?

you can see the given figure is example of cascade system before going to why we need cascade let us know about what is meant by cascade  system. when output of one transistor is given as input to other transistor or series connection of transistors  is called cascade connection.
Now coming to the real question why we require cascade connection. Cascade system gives following advantages
1.It increases  the overall gain
2.Sometime the input impendence or output impendence will be different from what we want then we use cascade connection and make desired values