Why we are using iostream at here, where as in turbo c++ compiler, we use it as iostream. h?

Answer 1

Because it is simply 2017, and Turbo C++ was last updated in 1996.
C++ has come a long way from that compiler...

This question translates to :

Why use Windows 10 when we have Windows NT?

Answer 2

Here's the true reason...

members.gamedev.net/sicrane/articles/iostream.html