[icebraining]

Nobody says you should, it's purely a personal preference :)

Doesn't one language has a purpose of it's own?

Usually no, most domains are served by more than one language, otherwise we would all be using Perl to write server-side web applications.

While it's debatable whether JavaScript is particularly well suited for client-side web code, the fact is that having only one language of choice is mostly an historical accident.