Why they use the header file iostream predominantly, here? Why not iostream.h?

Answer 1

https://stackoverflow.com/questions/2976477/difference-between-iostream-and-iostream-h

iostream.h was never part of the C++ standard, but it was used by early compilers. The standard C++ include files don't have ".h" in their name. THIS IS NOT C!!

Answer 2

For faster compilation

Answer 3

For standard namespaces and for new compilers like Dev c++ use without.h in big project without.h matters because it will compile fast