Is it necessary to use virtualenv to use Flask framework?
No, there is no requirement to use a virtualenv. No project ever would require you to use one; it is just a method of insulating a collection of Python libraries from other projects.
I personally do strongly recommend you use a virtualenv, because it makes it much, much easier to swap out versions of libraries and not affect other Python projects.
Without a virtualenv, you just continue with installing the dependencies, and they'll end up in your system libraries collection. Installing the Flask project with pip will pull in a few other packages, such as Werkzeug, Jinja2 and itsdangerous. These would all be installed 'globally'.